Smoker joining theory from a year ago (inside spoilers):
! What originally drew me to Smoker's character and made me feel he could potentially join the crew was his interactions with the Straw Hats back at Alabasta. The moment Zoro saved Smoker and Luffy told him he kinda liked him seems significant to me. At the same time, Smoker not only allows Luffy to escape, but trusts him to take care of business with Crocodile. Smoker later tells the higher ups to eat shit when they give him credit for the Straw Hats' victory against Baroque Works. The thing about Smoker is that he deviates pretty far from the standard marine. I'm not just talking about the corrupt ones like Vergo, who don't represent the Marines as a whole, but even the rank and file marines like Koby and even Tashigi who follow their orders. Smoker bucks the system and goes his own way, following his own sense of justice. While it doesn't make him a villain, it certainly makes him atypical for a marine, expected to respect authority and follow orders. He's gotten away with this in the past, but it certainly holds him back in some ways that his attitude does not mesh well with Marine authority. Marines are not the only force capable of fighting for justice as we've come to seen, so I think even from this perspective Smoker may be better suited to fighting injustice from a position more in line with his attitude and philosophy toward justice.
! Similarly, we've seen that the Straw Hats are atypical for a pirate crew. They aren't blood thirsty murderers who pillage and plunder. They're more like a happy band of outlaw vigilantes on a quest for treasure. They meet and befriend people everywhere they go. Everyone from pirates to kings seem to develop friendships with the crew and come to respect what they stand for. Much like Smoker, the Straw Hats fight for true justice. True enough, at this point they aren't trying to save the world (yet). However, they do fight for justice wherever they go in order to protect those in front of them. Consequently, though he set out from Logue Town in pursuit of Luffy, it seems every time they meet up they end up fighting on the same side. This simply indicates that Smoker and the Straw Hats hold a similar view of justice. Interestingly, on both occasions they end up fighting together against forces sanctioned by the world government (Warlord Crocodile back in Alabasta; Vice Admiral Vergo and Warlord Doflamingo in the current arc).
! Another point to consider is that Koby is likely to play an important role in the story. Thematically, Luffy really only needs one true 'rival' in the marines. I just don't see Smoker as "Luffy's Garp" as some have described him. It just has a lot more weight from a story telling perspective to have Luffy's friendly rival Koby rise to a high rank in the marines.
! I think that its' likely we'll see the marines reformed at some point, especially now that Akainu has taken charge. This all makes sense when you consider that Luffy will someday fight against the World Government. Yes, certainly once the WG comes tumbling down, the good marines and leaders will take over. We won't see total anarchy, but reform of the system. This may mean complete destruction of the WG and total autonomy of individual countries or it may mean the Marines and WG are rebuilt from the ground up. Regardless of what exactly happens in the end, it's not going to be a smooth transition to get to this point. With piracy on the rise and a militant leader who serves Absolute Justice in charge of the Marines it seems that a storm is on the horizon. Akainu's absolute Justice does not mix well with Smoker's tendency to go against authority. This worked for Garp under the more lax leadership of Sengoku. It also didn't hurt that Sengoku and Garp fought alongside eachother and remained personal friends. Smoker has no such relationship with Akainu….I'm not sure that anyone does. We've already seen Koby stand up to Akainu back at Marineford. Thematically it would be better to keep Koby as the marine fighting to reform the organization and serve true justice from within it's ranks than it would for Smoker to fill this role. Koby already stood up to Akainu once. Wouldn't it be fitting for him also to be the marine to stand up to Akainu as he rises in strength and rank without going against authority. Koby is younger than Smoker and growing at a faster rate. He went from a wimpy cabin boy on Alvida's ship to a marine capable of using soru and developing haki just from the beginning of the story to the time skip. In two years, with haki awakened it seems he'll grow even faster. Koby and Luffy set out on their respective journies at the same time. Thus Koby is well suited to be Luffy's marine 'rival'. They seem to have a friendly respect for each other similar to Roger and Garp. I don't think their relationship will be exactly the same because Luffy's journey will not be the same as Roger's. After all, this is Luffy's story, not Roger's and all foreshadowing indicates their stories will end in completely different ways. So if Koby fills this role, what would be left for Smoker within the marines?
! At this point, we're almost 700 chapters into the series. In the 600 chapters since Smoker began chasing Luffy they've fought on the same side in two major incidents and have yet to seriously fight each other simply because they have similar views of justice. Thus it just doesn't seem right to have Smoker serve as a marine who constantly fights against Luffy in the New World. Koby is likely to rise to a higher rank as his strength increases.
! I think we're being set up for Smoker and Tashigi to be separated. Back in Alabasta, Smoker told Tashigi she had to find her own sense of justice and get stronger. When he departed with her at Punk Hazard to go settle things with Vergo, the tone of the conversation seemed to hint at Tashigi taking charge and Smoker somehow being removed from his current position of authority over G5. I think it's possible that should Smoker either leave the marines or be forced out, Tashigi will remain or will depart on her own. I think that this will be a better fit for her character arc as the figure she previously viewed as one of the foremost beacons of justice in her life is either knocked down or deviates from the path she would expect. Her developing respect for the Straw Hats would at least help ease this transition for her in accepting Smoker's new affiliation while she continues along her own path, striving for justice either within the Marines or with the rest of G5. I personally think it's more likely for her to remain within the marines while smoker is ousted. Tashigi is more obedient and less powerful. She's less of a threat than Smoker, who is likely to be a thorn in the side of corrupt marine and world government officials if left in his Vice Admiral rank. If we look at Tashigi's role in the story, she's been constantly knocked down or defeated. At least story wise, it would actually be pretty fitting for her character to be forced into a position where she needs to grow both in strength and in resolve as the person she previously looked to as her guide and leader is now gone.
! Just looking at the marines as a whole:
! Garp / Sengoku have passed the guard on to the next generation.
! Akainu is miltant and asserts the marines' authority to commit injustice for the sake of the greater good. He follows absolute justice, meaning the ends justify the means.
! Koby is a true blue marine who listens to authority, but isn't afraid to speak his mind even if it means getting himself killed. He stood up to Akainu in a big way which changed the fate of the world by saving Luffy's life. Following his character from the beginning of the story to the end, we'd see him go from Cabin Boy to Marine Admiral and eventually the one to help reform the marines from within. This works well from a story telling perspective. He and Luffy have a close personal tie, so eventually having Koby as the foremost marine and Luffy as the foremost pirate will bring things full circle. If Smoker is no longer within the marine ranks, it better serves both their characters as we never have to see Koby eclipse him in terms of rank and strength. Having Koby challenge Akainu's orders by speaking his mind will also bring full circle his character arc from Marineford when he first stood up to Akainu. Having Koby rise to the rank of Admiral would really show Koby's personal growth. This just doesn't work for Smoker who has yet to interact with Akainu in any way shape or form. Koby has history with Akainu, which no doubt will play into any arguments / feuds the two may have. Akainu doesn't seem the type to forgive and forget, yet if Koby becomes truly strong he'll rise in rank. This sets things up nicely for when it comes time to take down Akainu. Smoker not only challenges authority, but completely goes against orders if it doesn't fit his view of justice. While Koby is the type to stand up for what's right, he doesn't have Smoker's rebellious streak. While rebellion and near outlaw behavior gets Smoker by at G5, it doesn't bode well for his chances to rise to the rank of admiral under the leadership of Akainu. I don't think Smoker can ever rise to the rank of Admiral. While Garp never attained this position either, we've already seen that Koby has aspirations to someday become an admiral. This would also tie in nicely as Garp was his teacher. He would surpass his teacher, thus becoming a truly strong marine. Luffy, the heir to Roger and Koby, the student of Garp having a similar rivalry / relationship with a different ending also works well. Koby will undoubtedly accomplish his goal. If Smoker is still in the marines, having Koby pass him sort of brushes him under the rug. If he's already out of the marines doing his own thing, it doesn't make it seem as if he's been eclipsed and helps his character remain relevant.
! Smoker, having fought for true justice often at the expense of breaking orders, may (likely) eventually find himself out of a job. At the very least, he's not going to rise any higher in rank with Akainu leading the marines. While Koby fights for justice within the marines ranks, it would seem Smoker's personality makes him best suited to fighting for justice and reform from outside of the Marines. I would think Smoker's goal / dream would be to reform the marines and fight world government corruption. We as readers know that the Straw Hats would be a good home for him as they will presumably fight this very injustice some day. Smoker's goals fall in line with Luffy's eventual role as the one to lead the charge against the current World Government in the great war. Smoker could fight for justice and reform alongside someone else, but no matter who he fights with it will be lesser than if he stays with the Straw Hats.
! In terms of strength, I feel Smoker has already been eclipsed by Luffy. He was curbstomped by Law and I can't help but think that Luffy and Law are on at least even footing. Even Doflamingo seems to believe Luffy is now stronger than Law. With Luffy, Sanji, and Zoro now capable of using Haki, I feel this places all three of monster trio at or above Smoker's level. Smoker's logia ability is not so overpowered as to overshadow the crew, especially with the likes of Zoro, Sanji, and Jimbei on board. Haki users will be able to hit him, and I'm assuming we'll see an increasing number as the Straw hats take on bigger and stronger opponents as the series progresses. Personally, I believe that Sanji and Zoro are both above Jimbei and Smoker at this point, simply because I feel the monster trio will be preserved. However, Smoker and Jimbei are both still powerhouses. Smoker is tough, but at this point I just can't help but feel that like Jimbei he's not growing as fast in strength as Luffy. Luffy's increasing at a faster rate, which makes sense. At 19 he's got a lot more potential for growth. I'm not suggesting either of them won't grow or will get weaker, simply that Smoker doesn't have the same potential. Thus another point against him being "Luffy's Garp". Interestingly, it also lends further support to Koby, who as a young marine has shown the potential to grow quickly. We've already seen him make exponential growth even if he wasn't caught up to Luffy at the time of Marineford, but he's set his eyes on the rank of Admiral just like Luffy's racing toward becoming the next Pirate King.
! As for his interaction with the crew, I don't know if anyone has watched Chuck, but if they have I see Smoker as a John Casey (Adam Baldwin) type. Often no-nonsense and serious as a trained soldier. However, he has his light side brought out by those around him. Casey was all about gunning down terrorists and bad guys, avoiding personal ties, and following orders. But his years with Chuck help him lighten up. I think we need to view characters as dynamic rather than static. Yes, Smoker hates pirates, but Luffy can help him see the light. Yes, he's all business and no fun. Luffy and the Straw Hats can help him loosen up and realize that the journey can be fun and fighting for those in front of them may be the truest quest for justice he's going to find. It all depends on how the story develops, but Smoker is a dynamic character. His views on pirates can change just like Nami's. It's clear that he must serve some important role in the story. He set out to chase down Luffy, yet he's fought alongside him both times they've met. He doesn't seem to have a role as Luffy's marine 'rival' since Koby fits so much better. So then what's left for Smoker? To go off and fight for true justice with Aokiji? With Dragon? At least he's spoken to both of these characters, but when we the readers know that above all other characters Luffy and the Straw Hats are likely to effect the greatest change on the World and it's current structure wouldn't it best suit Smoker's quest and his character to put him together with them? He's come to know them well and considering the current circumstances is likely to keep fighting alongside them. His last conversation with Tashigi and Vergo / Doflamingo's ties to the Government don't bode well for his future in the marines.
! As said before, I think this will leave Tashigi to fend for herself and find her own path of justice with her leader and mentor gone from the ranks of the Marines.
! In this scenario, Koby, Tashigi, and Smoker all have different journeys but ultimately never deviate from fighting for true justice. Each of the three characters grows in ways that fits well with the groundwork already seen in the story up until this point.
WARNING: AVATAR - THE LAST AIRBENDER SPOILERS AHEAD
Simply put, I think Smoker can be compared directly to Prince Zuko. Zuko's entire purpose in life was to capture the Avatar. We eventually learned that his intense desire was not based on who Aang was as a person, but on his family history, and his relationship with his father. Zuko came to realize his destiny was not to capture Aang, but to join him to help defeat Firelord Ozai. Smoker's desire to capture Luffy is simply due to his hatred for pirates and his memory of Roger's execution. For such a major character, we know virtually nothing of Smoker's past. I feel that in the future, we will learn that Smoker's desire to capture Luffy / prevent him from becoming Pirate King will be revealed to emerge from a tragic backstory involving pirates in which his intense hatred was born. From this, we'll see him shift his hatred away from Luffy as he realizes that the title pirate does not make Luffy a bad person, nor does the word marine stand for true justice. Smoker's always been independent and as such, I feel he'll come to realize the best way to follow the path of true justice is to join Luffy. I really would emphasize the relationship between Luffy / Smoker being very similar to that of Aang / Zuko.
I've basically been promoting this same theory for many years and after having recently watched Avatar: The Last Airbender (twice), I came to realize the Aang / Zuko relationship is a perfect example of what I'm expecting to happen with Luffy / Smoker.
Luffy telling Smoker he liked him back in Alabasta is analogous to the moment when Zuko saved Aang from Zhao as the Blue Spirit and Aang said that in a different world the two could have been friends. Zuko only saved Aang because Zhao was a greater enemy. Similarly, Smoker let Luffy and the Straw Hats escape to fight Crocodile. Compare the two scenes….they have striking similarity. The big difference here is that there's much more going on in One Piece, with a LOT more characters. Smoker's chase has been on the backburner where Zuko's chase remained in the forefront. Just as there was an internal conflict in Prince Zuko, there's a conflict between Smoker's ideals and those of the World Government / Current leadership of the Marines.
Often Smoker's hatred of pirates is cited as an absolute, unchangeable trait. But is that what truly defines him as a character? Smoker's hatred of pirates emerges from his desire for true justice. The reason I feel he will be forced to abandon or modify this belief is because it conflicts with his true character. What truly defines Smoker is a quest for justice. He seeks to fight against corruption in any capacity. He's already shown his intolerance of corruption within the Marines / World Government. The second facet of that character trait is to realize that not all pirates are evil.
Having already realized that not all marines are good, Smoker must come to terms with the fact that not all pirates are evil for his character arc to be complete. We've already seen him start to realize this in the arcs where he catches up with Luffy. Both times he's fought with Luffy against corrupt government officials or WG sanctioned pirates. Again, I bring up the example of Prince Zuko. His desire to capture the Avatar was not the core, unshakable aspect of his character. Zuko was driven by his desire to reclaim his honor.
Capturing Aang was simply a means to an end, as is capturing Luffy for Smoker. He felt that capturing the Avatar would bring him honor, through redemption in the eyes of Firelord Ozai. Similarly, bringing Luffy, the son of Dragon and heir to Roger, to justice would in Smoker's eyes, bring about true justice. But just as redemption in the eyes of Firelord Ozai did not offer Zuko the honor he was seeking, bringing Luffy to justice for the sake of the World Government will not bring about justice in the world. Zuko eventually came to see that the only path to truly reclaiming his honor was to join Aang in his quest to defeat Firelord Ozai.
Ultimately, Zuko's redemption came through helping Aang defeat Ozai and he was able to take his rightful place as the new Firelord. Similarly, I believe Smoker must come to terms with the fact that Luffy will ultimately be a beacon of true justice against the world Government, the greatest entity of injustice in the world. As such, his quest for true justice, the real essence of his character, will lead him to join Luffy.
Zuko and Smoker are both characters whose allegiance is at odds with their core beliefs / character and thus their goal / drive is misguided. Thus, they must change allegiance such that their core beliefs / character are in line with their main goal / drive in order for their character arc to be complete.
Zuko's entire purpose was to capture the Avatar. His loyalty was to Fire Lord Ozai and the Fire Nation. Zuko was an honorable character, and thus he was fundamentally at odds with the Fire Nation. Ultimately The Fire Lord could not restore his honor. It was only through joining Aang, who unlike the Fire Nation, was honorable and fought for peace, that Zuko was redeemed. Zuko's ideals were very closely aligned with Team Avatar than with his father and the rest of the Fire Nation.
Smoker's entire purpose is to capture the future Pirate King. His loyalty is to the World Government. Smoker is honorable and his true motivation is to seek true justice. This puts him at odds with the World Government, which is corrupt to its very core. Capturing Luffy would not bring about True Justice, but would ultimately further secure the World Government's power. Smoker can best serve true justice by joining Luffy, who unlike the World Government, fights for justice wherever he goes (at this point, he merely helps people by circumstance, but in the future, I believe that once he understands the true history he will set out to take down the World Gov). Smoker's core beliefs are more in line with the Straw Hats ideals than those of the World Government.
Beyond Smoker, I think the other potential candidates are Jimbei, Kinemon, Momonosuke, and Vivi
! Law is a captain of his own crew who would be left completely on their own if he left them. There's no real standout among his crew who could even conceivable take over as captain should he leave. Penguin and Sacchis' names are only given in SBS and Bepo hasn't done anything nearly impressive enough to be considered captain material. My belief is that Law and the Heart Pirates, like Hancock and the Kuja Pirates, is going to be a member of Luffy's group of permanent ally captains / crews who will come to his aid in the future. I will say that Law has taken a very central role in the story, which I think is both interesting and essential. I would like to see Oda continue to utilize Law as a main character, central to the story, frequently appearing beyond this arc even as he does not join the crew directly.
! Jimbei is a captain as well, however, he's made it clear he's willing to leave the Fishman / Sun Pirates to joint he Straw Hats. He's actually more than willing as he expressed his intention to do so with Neptune in private after his promise to Luffy that he would join if asked again in the future, citing his entanglement with Big Mom as the only obstacle currently preventing him from doing so. His status as captain of the Fishman pirates can't possibly be used as a reason for him not to join as Jimbei has already directly stated his intention to join the crew in the future. He promised Luffy he will join in the future. As a man of honor whose word is sacred, Jimbei does not make promises lightly. Unless we are to assume that Big Mom will maintain constant dominance over Fishman Island, his joining is inevitable.
! Vivi is a nakama. I believe she'll play a central role in the future, otherwise Oda would not have shown her to understand Luffy's message to his crew in 3D2Y, nor would he be setting her up for Reverie. I have my own thoughts as to how and why she could possibly reunite with thew crew as circumstances in Alabasta may not be as static as they seem. All that is pure conjecture and as I've gone into detail in the past about these ideas, they don't need to be restated. I will say that Vivi's status as a nakama and honorary Straw Hat goes without question. How she will be used in the future is certainly up for debate. Some believe that she has been permanently sidelined, but I feel she's only in reserve until the One Piece world changes to a point she must take action once again and be reunited with the Straw Hats, be it in the new world or in the big final war against the world gov.
! I don't think that Violet or Rebecca have much chance of joining, if any. Both of them are tied to the monarchy of Dressrosa. They're similar to Vivi in their connection to their country with even less connection to the Straw Hats. While Vivi was introduced 3 arcs prior to Alabasta and spent all that time with the Straw Hats building her relationship with the crew. It's an entirely different scenario from Rebecca and Violet, both of whom were only introduced in the arc central to their conflict. They are part of Dressrosa and were only introduced in the Dressrosa arc. Both of them are inextricably tied to the conflict in Dressrosa and should the Riku Family resume power, they both have a duty AND a desire to stay with them. Rebecca's "dream" as it were is to live happily with the Toy Soldier. Unlike Vivi, she harbors no desire for adventure. She has no strong connection to the Straw Hats. Vivi spent time with the entire crew and had a deep connection with the Straw Hats. Conversely, Rebecca only knows Luffy and has yet to meet any other members of thew crew. Rebecca's entire character revolves around Dressrosa, thus making her unsuitable to join the Straw Hats as things stand now. This was not true of Vivi, a multi-faceted character. Vivi had both obligation to Alabasta and desire to sail with the Straw Hats. She had a deep bond with her country and the entire SH crew. Due to circumstances, the former took precedent and thus she chose to remain in Alabasta. There is no such struggle in Rebecca. Violet is even less likely to join. Not only does she lack connection with any crew member aside from Sanji, she hasn't even MET Luffy. No member has ever joined after forming a strong bond with another Straw Hat before even meeting Luffy. While I have no problem with this pattern being broken, as things stand now, Violet has no chance of joining unless she forms a relationship with Luffy.
! I think Kinemon and Momonosuke have a distinct chance of joining. It will depend on how the story develops, but for the time being the fact that they both are developing bonds with the entire crew and are sailing with them for multiple arcs (with hints of a tragic(?) backstory), is at least something to be considered. Few characters hang around for multiple arcs and sail aboard Sunny. The last person to do so was Vivi and she's considered an honorary Straw Hat, a distinction not made for any other ally. The fact that they've even shown up in a few color spreads is also somewhat noteworthy. I'm not claiming any of this as absolute evidence of them joining, but it does make them potential candidates.
See the above for my thoughts on other potential crew members