
But in the beginning of the story, Luffy said he wanted 10 crewmates to join him. There's forums online discussing what position the new person would have. They currently have a captain, first mate, navigator, cook, doctor, gunner, shipwright, scholar/historian, musician and helmsman. They don't have a lookout or a quartermaster. A lookout makes more sense for the 10th crewmate since the quartermaster is just an extra. I think they should get someone that can fly since they currently dont have good air support.
Do I think Yamato or Carrot will join the crew? No. They've travelled with several people for long periods of time and then not included them in the crew (like Vivi). There's been no hints that Luffy wants to invite either of them.

I get you. I honestly feel the same way. There's supposed to be like 4-5 more years left (so 200-300 more chapters). Hopefully Jinbe and whoever joins next (if Oda doesnt change his mind about adding one more person) assimilates into the group before the end. Also, lately the crew is all split apart. There hasn't been many crew bonding chapters since Jinbe joined. It was more like going straight to war. He hasn't really been given a chance to truly become part of the group. He barely even appears.
As of now, the crew is at a solid ten members. Each one has an ambition and title: greatest swordsman, greatest navigator, greatest sniper, greatest cook, greatest doctor, greatest archaeologist, greatest shipwright, greatest musician, greatest helmsman, and of course, the Pirate King. What would Carrot and Yamato be? There really aren't any more core roles to be filled. I suppose they could be part of the fighting team, but not sure if their aspirations match those of the PK's crew.