Thing is, Oda already did do something unexpected and unpredictable with Yamato. When she straight up asked to join the crew it was like nothing we'd seen before. Sure Robin also asked, but it is the fact that this was one of the very first things that Yamato said to Luffy and the audience that makes it so surprising. But he did something smart here and made it so that there was no straight answer from Luffy plus made it so they went separate ways. This scene builds anticipation and expectation for the future of the arc and now we're just waiting for the payoff. Honestly, this situation feels a bit of what happened with Brook.
The thought of Brook being a crewmember is planted in the audience's mind -> Brook's current circumstances prevent an official answer + they go their separate ways -> Anticipation and expectation is paid off when Brook joins
Say in an alternate world Brook decides to go off on his own to reunite with Laboon. Doing that would feel shitty as the audience was expecting a payoff with him joining. But what Oda does next is that he creates another set-up that builds anticipation and expectation for the future. Maybe he has Brook make a promise, maybe some ominous narration for the future comes up Grand Fleet Style. Either way, it allows the audience to easily accept not getting the payoff they expected.
And with Yamato:
The thought of Yamato being a crewmember is planted in the audience's mind -> Yamato's current circumstances prevent an official answer + they go their separate ways -> Anticipation and expectation is paid off when ???
In the same case here, if Oda decides to not go with the expected payoff then he needs to make another set-up for this character. If she's supposed to be Wano's guard mutt for the rest of the story I'm not sure how he's going to do that beyond a simple cameo in the final war.
So maturing and growing as a character is to give up lifelong dreams and listen to the whims of an abusive father? In the same series where this moment was a highlight?
What benefits would she add to a country beyond just her strength? Cause Wano already had a reputation that the samurai was strong as fuck to the point that AKAINU acknowledges them. Add Momo's current state + Luffy's influence + the fact that CP0 members as well as WG members, and TWO Yonko's and their crew might get kicked out of Wano tells me she's not really needed in this regard.
No story is invincible, it doesn't matter how popular a series is it's important to keep people engaged in reading weekly + getting them to want to reexperience your story. And considering that Yamato has had several chapters since the beginning where she's stated she wants to join/adventure I would say that the drive to meet fan's expectations comes from Oda here and not from outside forces.
Like, I agree that the big thing that Yamato is lacking is interactions with the crew. But I feel that it's a problem that's quickly fixable (And I think Oda is in the middle of fixing it with Brook and Robin). It's honestly the same with some of the major issues that Carrot has. Lack of relevance? Give her a hype redeeming moment.
But considering Yamato's relevance and popularity in the story I would say that she's going to get a proper send-off if she doesn't join. Yes, she came out of nowhere but she has since earned her place in the story. Also, I point to the Supernova, who were created right before Saboody and have influenced the story in major ways since.
Finally, sure Carrot
could do the job but not with her current situation. Has been mostly absent since 1006 and the strongest memory in the community is her loss + Nekomamushi stealing her fight. She'll need that final push to get to that point.