Ok I've been a soft Bonney supporter for a very long time, and now that she is a major character, I have very mixed feelings about it. There are just a whole bunch of arguments for and against her joining the SHs, so I'll list all that I could think of:
For: She just got one of the best flashbacks in One Piece, complete with a sad story, a (mostly) dead mentor who is very likable and a villain who is also the guy Luffy is most likely about to beat.
Against: She has no dream to speak of. Her only known dreams so far have been to find her father (fulfilled as of 1103), and to avenge her father (likely to be fulfilled in this arc, also not a "good guy's" dream). She needs a new one, a reason to want to sail with the SHs.
For: Kuma's monologue at the end of the flashback all but stated Bonney is a major character, someone who will be integral to defeating the WG. So she is extremely likely to stay in the story until the end.
-> Counterpoint: She could also accomplish the above by joining the Revs instead of the SH. Though a counterpoint to that is that she has no means to reach the Revs right now, unless they go to Egghead. And if they don't, she is 99% likely to be with the SHs at least until Elbaf, since that's the only known way for her to escape the ongoing buster call.
Against: Kuma could survive and maybe even go back to being a human. So she would have someone to go back to, similarly to Rebecca.
-> Counterpoint: Look at the man, it really seems like he is on his last ropes. And his monologue in the flashback heavily implied this is a "passing the torch" scenario. Also as much as Oda likes to undermine his own writing by undoing the tragedies in the story, it would be better for the story if he did not.
For: She has a possible role, Apprentice, which has been seen in other major pirate crews, most famously Roger's. Marco and Kaido were also apprentices in WB/Rocks crew when young.
-> Counterpoint: An apprentice who became a pirate at the same time as Luffy and arrived with him in Sabaody is not really an apprentice, so it would be in-name-only.
Against: She already has a crew.
-> Counterpoint: The flashback did reveal her crew is more of an improvised bunch of townies who wanted to help her than actual pirates. And after the scene at the end of the timeskip, it's very likely they are in prison.
-> Counterpoint 2: That didn't stop Jinbe.
For: Finally another female character, which many people agree with is good since it balances the crew. And she is very solid in that regard for a few reasons: not another sneaky assassin like nami/Robin so she feels fresh, and she has some of the most distinct female faces in the OP cast (that kid lipstick and the angry frown are impossible to mistake), which is relevant given what we are used from Oda regarding female faces.
For: Unique hair color. Not that important, but no hair colors in the crew have repeared besides black.
For: She has a major conflict in the arc, related to her sad past, that needs solving, most likely to be done by Luffy. This has been a constant in the crew, with strawhats always been saved by Luffy and being given a new onlook in life after meeting him.
For: She already worships Luffy, though she doesn't know yet. Also she is getting bonding scenes with Luffy and the crew. And he even calls her by name!
For: She can fight at a similar level as most strawhats, and is not strong enough to outshine the M3. Her fighting style has some variety too.
Against: She is too much of a Luffy clone. While she has fun abilities, her main move consists of imitating Luffy's stretching powers, and once she sees Nika in action, she may just continue to add more Nika moves to her arsenal. She also shares the big eater role with him. That is bad for manga variety.
-> Counterpoint: Shanks was/is a complete Roger clone in terms of fighting style, so there is precedent. Although there wasn't a manga about the Roger pirates for that to harm variety.
-> Counterpoint 2: There is also precedent of SHs repeating quirks from previous ones, such as Brook repeating Zoro's swordsmanship and Sanji's pervertness, Franky being a second tech guy, Jinbe being a second responsible for saving drowning strawhats after Sanji. Though I'd argue none of those are as overt as Bonney's imitation of Nika.
For: She has bones to pick with the Gorosei, Blackbeard and Akainu! Not hard to make Bonney care about all the upcoming conflicts.
For: If Oda ever decides to have Luffy give the strawhat to a successor, she seems like a natural candidate.
-> Counterpoint: She doesn't necessarily have to join the SH crew for that.
Against: It's WAY TO LATE in the story for a new SH. Yes, Jinbe also technically joined super late (WCI or Wano, depending on what qualifies as joining), but he had most of his development between ID and FI. it was really just Oda sneakly introducing him into the crew without letting him break the M3. Bonney's panel time is pathetic compared to all the previous SHs, and there is not enough manga left for her to really shine as a major character. Also she missed Luffy's dream reveal.
-> Counterpoint: She has technically also been in the story since before the timeskip. Heck, her first appearance predates Jinbe's, so the above is not completely true.
Against: According to Greg, the original crew pre-serialization was 10 people including Luffy, so this would have to mean Oda had some change of plans. While that's not unfeasible, would be a first regarding strawhats. To add another issue, we know Bonney was created overnight without much thought, as part of the Supernova batch.
-> Counterpoint: It's likely Oda had a concept in mind for Kuma's daughter before, and he then decided to fit Bonney into that. He did a similar thing with Vivi, and she ended up being the closest thing to a strawhat any non-strawhat has even been.
For: Luffy said he wanted 10 crawmates. If it's not her, then who? Each arc we go without a new strawhat greatly reduces the likehood of anyone joining.
-> Counterpoint: That may or may not include Luffy himself, is clearly not a precise statement by him, and doesn't need to be treated as an author statetement on the final crew size.
For: 11 Blackbeards vs 11 Strawhats.
-> Counterpoint: The reveal that Aokiji is one of the 10 Titanic Captains greatly complicates the match-ups. Not only he is being hinted at being a mole, there is also the fact he competes in both power and character importance with Blackbeard. Traditionally, subordinates of villain groups follow a pecking order, to match the SHs. He doesn't fit as a Zoro opponent either, and would be weird to have Sanji or Jinbe fight someone who is close in level to Luffy's opponent and clearly above Zoro's. If he doesn't outright leave the crew, it looks more likely he will also be fought by Luffy too, or by an outsider ally such as Koby and/or Smoker. That means 10 blackbeards vs 10 strawhats is making more sense.
-> Then again, just because that is cool it doesn't mean every SH needs to fight a BB.
Conclusion: Eh... personally I'm not feeling it, and lately every time I really felt it it turned out to be a dud. But there are enough solid arguments for it. I'm gonna give it a 30% chance Bonney joins. I like her way more than I ever did Carrot, and also more than Yamato after the midpoint of Onigashima when it started looking Oda had nothing interesting to do with him. Bonney is VERY interesting, there is care being done in her writing, I'm just not completely sure what for.