I'm interested to see what new characters would join the crew, but I really think they need to fill a role. Almost all of the crewmembers joined after the crew said, 'We need a ___'. Then they'd have to convince the person they conveniently found next to join, because they needed that role to be filled. If someone joined with no immediate purpose, they soon show they have the skills to perform a certain role. Zoro's special, 'cause he's the first mate, but Usopp joins just 'cause he and the rest of the crew were close now. Soon after that, he's officially named the sharpshooter after he shows skills in that area. Robin asked to join the crew and said she was an archaeologist and soon after proved she was a reliable source of information. Brook was asked to join just 'cause Luffy thought he was interesting, but then showed he was a musician, whom Luffy had wanted since day one, which makes him determined to get him to join.
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.