What I'm trying to say is mostly crewmembers join when the rest of the crew specifically say that they need a role to be filled. If they don't hold that 'meeting' beforehand, then the crewmember that joined shows some sort of skill so they are assigned a role. I'm trying to think up possible roles to fill if Jinbei or Hancock or whoever joined the crew, but I'm drawing up a blank. We don't even need a specific helmsman anymore because Franky or someone else just use the wheel. This may be a bad example, but Vivi was looking like she would join the crew but stayed behind for her country. (Kinda a relief for me, but anyway…) I sat down and thought about it. If Vivi had joined, how would she be useful? She has no specific skills except for maybe public speaking.... which would not be useful at all. She isn't very good at fighting either. Whenever she had to fight, she would always come off as pathetic, so that's a no-go. She would just be baggage. Now I know, the crew is close to her and they would love to have her on the chip even if she was useless, but they're current mindset would be 'Can we really afford that if we're gonna be going to the New World?' Especially with that huge Wham Episode we just went through, I'm thinking we're gonna need a useful person that can take care of themselves. And since there are no current roles to be filled, I doubt someone's gonna join any time soon.
I googled 'pirate ship positions' and got a list. In terms of the SH crew, we filled up pretty much all of them.
The Captain - Luffy
Quartermaster - Nami, Zoro
First Mate - Zoro
Boatswain (keeps crew in place) - Nami
Cabin Boy - _____
Carpenter - Franky
Gunner/Sharpshooter - Usopp
Cook - Sanji
Pilot - Nami
Sea Artist (rather, just an artist, draws cool things, sometimes maps)- _____
Striker/Hunters - _______
Surgeon - Chopper
Musician - Brook
As you can see, they already filled all the important positions. Robin's position of historian isn't really a position, but something that Oda deemed important. I'm sure he can fill character/Hancock/Jinbei to a role, or create one especially for them. Kind of like Robin.