You can't have characters join the SHs when they don't contribute anything for the crew, I just don't understand this fanboyish stuff.
Every SH has a fulfilling role:
Luffy – Captain
Zoro – Right hand man, swordsman (samurai)
Sanji – Chef
Nami – Navigator
Usopp – Sniper
Chopper – Doctor
Robin – Historian
Franky – Shipwright
Brook – Musician
Jinbei – Helmsman
Every other character mentioned like Carrot, Law, Tama, Hiyori, Kin’emon, Pudding, Momonosuke, Smoker falls out from the competition because they don’t fulfil any specific role to join the SH’s, especially Law and Carrot.
Although I’m not into Yamato’s character yet, and I don’t what role she fulfils (I haven’t caught up with the manga), but it’s obvious that she’s the next SH to join. The problem with Yamato’s character first and foremost is her name, and trying to be someone who she isn’t, in this case a man. So, I guess her role would be a front fighter or some sort.
Other far better candidates to join the SH’s who do fulfil specific roles are:
Monet – Astronomer
Caesar Clown – Scientist
Another least possible candidate to join the SHs would be Boa Hancock as a front fighter like Yamato and the reason would be her “love sickness” for Luffy. Even though I like her to join, the possibilities are very low and the only way I see her is accompanying Luffy is through his journeys as an ally who will anytime come to help him with the Kuja Pirates.