All of this was pretty obvious. And it certainly feels like Oda made Yamato to join from the get go and was all intentional.
Oda has done this with a majority of the Straw Hats who joined.
The other thing too is that for a majority of the Straw hats who joined, Luffy met them first and alone. The only exceptions are: Sanji, Ussop and Brook. But even then Luffy was quick to ask them to join almost immediately upon meeting.
Another criteria i would also add is that those who join are outcasts or who don't fit in with the crowd that they had previously belonged to before joining. Everybody who joined the straw hats pirates crew became a second home and family for them, whether it was out of convenience or otherwise. Zoro was forced to join or he'd get executed, Nami joined cause she didn't feel like home in neither the Arlong Pirates or CoCo Village, Ussop was the village liar that people didn't like, Sanji was the cook who didn't get along with his peers, Chopper was the snow monster, Robin was a fugitive on the run, Franky didn't have a place in Water Seven that would let him be himself, Brook was all alone on a ghost ship and Jinbe was a pirate who would always be discriminated against on the human surface world.
Yamato has no friends in Wano, and her only family is just a alcoholic abusive mess of a father.
Yamato definitely feels like Oda really pushing for a character to join in a long time. The last one was Brook, we didn't know if Jinbe was gonna join or not until after Fishman island until Luffy finally asked him.
I'm more curious as to why some say that she won't join but can't give good reasons why. I have yet to see a convincing reason why. Other than Yamato there hasn't been a more fitting candidate in the Wano Arc. There has also been those that say that yaamto will die, but i also think that won't happen, for the same reasons why jinbe didn't die on WCI.