Luffy, Zoro, Nami, Usopp, Sanji, Chopper, Robin, Franky, Brook
Jimbei
Kinemon
Vivi
Karoo
Vegapunk's Dragon
Smoker
More details later. I'm aware how crazy the Smoker theory sounds, but I've just had this feeling about him since the moment Luffy told Zoro to save him in Alabasta…then told Smoker he kind of likes him. Smoker is far from a normal marine and Luffy is far from a normal pirate. I don't want this post to be deleted so I'm going to edit it with more details momentarily. I don't mean this to be an off-the wall suggestion. I have reasons for thinking this....and most won't agree lol. I don't expect people to, but I do want to at least post it in detail.
Alright now the details for why I think each has a decent shot of joining.
Jimbei I've elaborated on many times before in this topic. It's all but certain to me that he's going to join. Started to get the feeling at the end of Impel Down when Jimbei called the whale sharks and Luffy said "Woah he's awesome"... it was so similar to Luffy meeting Brook for the first time and deciding he must be part of the crew.
Kinemon
I was originall against him joining the crew, but through this arc he's grown on me quite a bit. His fighting style actually compliments Zoro and Brook rather well even though at first it seemed he would overlap with Zoro. I definitely think there is more to his devil fruit ability. The thing that's starting to convince me of him joining is the fact that we haven't seen Momonosuke yet. I mean, here we are 26 chapters into the arc, yet we haven't seen Momonosuke appear even once. We haven't seen anyone but Kinemon even mention him. His absence is pretty conspicuous in my opinion…it indicates that there's some sort of twist here. We don't even know Momonosuke's age. We've seen his personality fits really well with the crew and drawn along side them he just looks right. I don't think that there's enough to say definitely by a long shot. It's hard to even make a case for him at this point because there's still so much mystery to him - but it's that mystery that actually gives him the best chance in my opinion. I would say possibly that Momonsuke is a villain - either here on Punk Hazard or back on Wano. For all we know, he's working for Doflamingo.
Vegapunk's Dragon
Okay there's no real basis here other than it's freakin' awesome. It's both really cool and kinda cute at the same time. It's the right size to fit on Sunny and I think Oda must have introduced it (word box and all) for some kind of purpose. Even Nami said it was cute. It would be nice to see Chopper acknowledged as the crew's doctor and receive a real bounty at some point in the future (transition from "Cotton Candy Lover" to "Monster" Chopper). Part of Chopper's character is that he secretly (and not so secretly) wants to be recognized as a man as much as a reindeer….so I think it would be a big character moment for him to get a real bounty. Oda gave him the title of the crew's pet. He has a funny way of hinting at things before they happen. I could see the crew picking up a real pet at some point and what better than a badass cute little dragon?
Vivi and Karoo
We've caught up with Vivi two times in the past. Once in volume 45 after Enies Lobby she was shown among a variety of supporting characters, some essential like Smoker, and others simply friends from past arcs like Nojiko, Zefff, etc. They were all reacting to Luffy's actions at Enies Lobby. Vivi was nothing more than a spectator at this point.
However, after the war she was shown in a different light. All the characters appearing were big name characters important for the New World; Crocodile, Aokiji, Smoker, Koby, Buggy, Doflamingo, Kid, Law, etc. Vivi was the only one who directly commented on the actual message; Luffy's tattoo.
She parted ways with the Straw Hats with the promise that they would call her their shipmate or nakama if they ever meet again. Oda doesn't tend to treat these moments lightly. Vivi left Alabasta once for the sake of saving her kingdom and would do so again if Alabasta was in trouble. World Government authority and Marine presence overruling her ailing father? Straw Hats reuniting with Vivi at Reverie? I mean, the last story arc showed Vivi is getting ready for Reverie and Cobra is sick. We also know Alabasta is where Pluton is seemingly hidden. I think at this point there are many signs Vivi will have a more important role in the story than just sitting on the sidelines. After all these years Oda still puts Vivi and Karoo on Straw Hat chapter cover sets (between cover arcs he does covers of just the Straw Hats doing random stuff and always includes both Vivi and Karoo). Unlike Paulie, Vivi is an honorary member of the crew and I don't think that should be taken lightly.
What if at Reverie it's decided that since Cobra is sick, this is a good opportunity for the World Government to put the monarchy out of power and take control of the country which seemingly hides Pluton (Crocodile was somehow privy to this knowledge, so I could see others also being aware of this rumor).
I mean there have been lots and lots of hints that Reverie would play an important role later in the story. The cover story arcs always lead to something. If anything could be taken from "The Decks of the World" it's that Reverie is happening soon. Oda sets up a lot of future arcs with his cover stories.
Smoker
I'm aware of how controversial this suggestion will be. However, I've had a feeling about Smoker since very early on….that he sort of fits with the crew. I'm aware that he's a marine and he hates pirates - so did Nami. Zoro hunted pirates. Nami stole from pirates. The early crew members weren't exactly pirates or anything close before joining.
I think it would be awesome to have an ex-marine crew member. A logia fruit user would also compliment the crew well and Smoker's is not so overpowered especially now that Luffy, Zoro, and Sanji can use haki. Luffy never stood a chance against smoker pre-time skip. I suspect we'll see their fight still, but after Law completely curb stomped him, I really don't think he's the powerhouse we imagine him to be. Many say that Smoker is "Luffy's Garp", his marine rival. I can see why this is said, but Luffy also has Koby. I don't see a reason story-wise for Luffy to have two primary Marine rivals. Smoker sort of overshadows Koby for sure, but with him developing Haki just before the time skip, I could see him as a force to be reckoned with later in the series. Just as Luffy set his eyes on Pirate King, Koby set his eyes on becoming a marine admiral. I believe that he will achieve this. I see Koby as Luffy's true marine rival so this leaves Smoker in an odd position.
We've seen Smoker meet up with the Straw Hats twice since he set out to chase them from Logue Town - in both instances he's ended up fighting on the same side as them. I think this shouldn't be overlooked. Smoker fights for true justice, yet he bucks authority on many occasions. At the end of Alabasta he told the higher ups to go eat shit. He's not exactly a conventional marine. Similarly we have Luffy, a pirate who fights on the side of justice for both his nakama and others. I think that this current situation with Vergo and Doflamingo could lead to Smoker either leaving or being expelled from the Marines in the future. Vergo could blame this incident on him. Smoker could try having Vergo removed from authority to no avail. There's many ways it could go down.
Then there's the question, "what about Tashigi?". I think back to Alabasta when Smoker told Tashigi she needs to find her own sense of justice. He will follow his own path and part ways with her on a similar note; follow her own path.
For instance, I think there's a possibility for a G5 arc in the future given that we saw taht other captain on a New World island.
In two situations now, Smoker has seen the Straw Hats fighting on the side of justice while other World Government and Marine allies fight against him (crocodile, one of the seven warlords and Vergo, vice admiral and commander of G5). I think that it's possible in the future Smoker will contrast the "pirate" Straw Hat Luffy with the so-called forces of justice and realize that Luffy's brand of justice is more effective. If he realizes he can't fight for justice within the marines and leaves or is expelled from the marines due to a conspiracy by Vergo or others who feel he's a thorn in their side, I could see him aligning himself with the Straw Hats. Smoker is so far from the typical marine and Luffy is so far from the typical pirate they actually meet in a very similar middle ground. Some sort of flashback to flesh out his character - maybe delve into why he's so driven to catch Luffy - could bring him around to the side of the Straw Hats.
Luffy even said he likes Smoker midway through Alabasta when Zoro saved him "on captain's orders" and Smoker trusted them to go take down Crocodile. To me that was a significant moment for both Smoker and Luffy. I know this isn't much evidence, but it reminded me of how he looked at other crew members before they were asked to join.
Strrength wise, I'm confident that not only Luffy, but also Zoro and Sanji are stronger than Smoker at this point. We've sseen him beaten down pretty thoroughly by Law and held off by Hancock in the war. He's strong for sure, but at this point I believe the "Monster Trio" to all be stronger than Jimbei. That leaves Smoker in a pretty precarious position as "Luffy's Garp" ... if he's so outclassed in strength, how can he be Luffy's main Marine rival? I think that role is best filled by the much younger Koby who has greater potential to increase in strength.