Well here's a place I thought I'd never come back to, but I just couldn't help myself. Yamato's odds for joining are such a fascinating subject, and you people are, admittedly, the smartest I know in any One Piece related matters. So I tought I'd give my two cents and see if any of you had something to say.
Frankly speaking… I really have no idea on what Oda's planning for Yamato. On the surface she seems like a solid candidate for crew. She's expressed her interest in joining more than once (and setting out to sea in general), she's gotten a lot of focus, she's shown great chemistry with Luffy...
But there's some pretty big issues about the idea of her joining as well, and I'm having a hard time to think a writer of Oda's caliber would have let so many to pile up. I can divide my concerns into four main issues. The first two are about something Yamato lacks, and while she could in theory acquire them later, I can't help thinking Oda could've easily given them already. The other two are about character aspects she already has, which feel rather contradictory for a crewmate candidate, and they can't really be rid of either:
1. She doesn't have a role for the crew. So far Yamato has shown no skills that would make her irreplaceable for the crew. She has only shown to be good with fighting. Now, a lot of people have said that she could be simply a fighter, but I doubt that. Every other Straw Hat can already hold their own in battle. Also, they're all specialists whose main job no one in the crew can do better. Luffy is always going to be the best fighter, so Yamato couldn't be one. And while there are some fighting oriented roles in the crew with Zoro and Usopp, they're also both specialized, in blades and long-range respectively. I have yet to hear about a maceman or ice-breather's positions.
Another popular theory I've heard is that she could be a logbook keeper, but I doubt that as well. It would be rather silly to start a logbook at this point of the journey, and Luffy probably wouldn't be interested in that anyway. Plus it wouldn't be a very useful role they absolutely need to fullfill. The only crewmate who can be considered a luxury is Brook, and even then a musician is something Luffy wanted for a long time, something he considers integral for his vision about pirate life.
Apparently there has been some talk about her becoming an advisor with Oden's logbook, and franky speaking that's just bogus! This is Luffy we're talking about. For anyone needing a reminder, he straight up said he'd quit piracy if Rayleigh revealed him the truth about One Piece, because "he doesn't want a boring adventure". He'd never want that kind of advice. Keep this point in mind, it will be important later on.
2. She hasn't had much interaction with the crew. While she did bond with Luffy a little at first, and as I said their interactions were fun, their time together was fairly limited. Only 7 chapters, and after that a brief talk with Franky. That was about it before the latest chapter If she is to join, why wouldn't Oda give her chances for little more interactions with SHs, to make her eventual inclusion feel natural? And if he's planning that for later, just how much longer is this arc going to be? She does have a chance to interact with Luffy more now, of course, but that still leaves the other Straw Hats out of the loop. One might argue they'll also join to take out Kaidou (which I doubt since Luffy has been taken enough credit of defeating him as is), but even then there's only so much bonding you can do with nine people in the heat of battle.
3. Traveling to Laugh Tale might be counterproductive for her supposed future character development. Now, I think it's safe to assume that Yamato will sooner or later discard the Oden act and become her own person. But that kind of begs the question: If she gets rid of "Oden's shadow" only to go on a journey with the same destination as Oden did, then would it really feel like she developed? Now, one could argue that staying to protect Wano (or maybe Momonosuke) and setting out to sea without clear destination would also be "Living in Oden's shadow", since Oden did all of those things at some point of his life. And that's fair. But journeying to Laugh Tale was neverthless Oden's "Magnum Opus" more or less, and likely the climax of his logbook. So I can't help but think that it would be just about the most "Oden" thing Yamato could do. Surely Oda could see the irony.
Then again, maybe Oda just doesn't mind. The inherited will is one of the central themes in One Piece, even if Yamato has taken it to borderline mental levels, so maybe he's fine with letting her continue on the path of Oden. Maybe she doesn't even give up on the Oden thing.
4. (And this is something we actually learned even before her sex. One page before in fact.) She knows too much about the big mysteries. It has been hinted a few times that Oden's logbook contains a lot of important information about things like Laugh Tale, Will of the D, just about every big question about the series. Not to mention the islands ahead, tough SHs could just take another path. Neverthless, it would eventually lead to Laugh Tale and quite possibly Lodestarr. The thing is, I'm having a really, really hard time believing Oda would let one of the Straw Hats have this much knowledge of the endgame. As I mentioned earlier, Luffy would never want that kind of info. There might be some exposition once this battle is over, but obviously we ain't gonna know everything.
Now one might say that Yamato isn't obliged to share her info with the Straw Hats, and true, she isn't. But even if Luffy and other Straw Hats wouldn't privy on the details, I think having even one crewmember know answers to these big mysteries would severely undermine the theme of adventure this series is built on. Luffy's refusal to hear about One Piece isn't just his personal conviction. It is a declaration of that theme! These mysteries have been tickling the back of our minds for nearly two decades now, and we've followed the crew all this time on their path to the last island, where answers are finally given. Aren't they supposed to find these answers the same time we do, so we can relate to their feelings at that very moment? Wouldn't even one crewmember knowing them already just distill that feeling?
I've also heard some people saying that Oden might not have even written that much about the mysteries. And while we don't indeed know yet how much he wrote, I'm still pretty sure what was written is quite juicy, based on how Yamato and Momo talk about it. Yamato has mentioned "important things" and "D" already, and Momo has realized something even bigger than Wano's fate about Kozuki's legacy. And no matter how little Oden might have ultimately written, we already know he already wrote about something Luffy would never want to know beforehand:
One Piece is something hilarious!
Now, this might seem like a small thing, but it's still a description, however vague, about the nature of the very mystery, the dream, or romance if you will, that this series is named after. This is the main quest for Luffy's journey, a mythical treasure on a mysterious island no one except Roger's crew has visited in centuries. As I've already said, even one Straw Hat having this kind of insider info about One Piece… Well, I have a hard time believing Oda would be okay with that!
Now one thing that might complicate the previous point is Whitebeard's declaration of One Piece being real. I doubt Luffy still knows this, as he's not exactly well-informed. But the other Straw Hats quite possibly do due to reading the news about the war. So, admittedly SHs might be on unequal grounds on what they know as is.
Still, for what it's worth, WBs reaffirmation about the treasure's existence did make it general knowledge. Whereas it being lulzy, and any other secrets Oden wrote, are still exclusive insider leaks. So take what you will about it.
Anyway, those are my main issues. I do have some smaller ones like her namiface, her design overall not being what I'd necessarily assume Oda worked a long time with, lack of foreshadowing + appearing pretty late in this arc (since Onigashima bit is still apparently part of Wano), her past being played in smaller bits instead as one big emotially charged flashback...
Now, with this thread having so long I'm sure at least some of these points have been discussed already. And perhaps you have already come up with answers I can find satisfactory, thought up angles I hadn't. I'm definitely curious to hear if you have anything to say.
Again, I wish to emphasize that I'm not denying Yamato joining! I'm just not sure at all about it! The focus she's gotten vs the qualifications I've mentioned, man, getting some really mixed signals. But maybe you folks or Oda will adress these issues later on, even the ones I doubted could be adressed.