So this is what I get from all these threads about who the next member's gonna be:
Viola is very likely the next in line for the throne, which is very likely to happen sooner than later, considering how Riku meant he wasn't worthy enough to regain the title as king. Even if she wasn't the next in line, I think she cares too much for her people to actually leave them. Her country needs her, especially after all the incident eight years ago.
Kyros and Rebecca just reunited with each other, so I'm pretty sure we can rule them both out by now. To even still considering them is beyond me, but I guess it's just about preference at this point. "I won't ever let my daughter hold a sword ever again!" basically says it all.
Bellamy's chances of joining aren't exactly as low as many make them to be, but personally, I think his pride is too big. He still fights for what he believes is right, and for some reason, he still wants to appear worthy in Doflamingo's eyes, which, at this point, is not gonna happen. Still, if he were to join someone's crew, I think it'd be Bartolomeo's, but who knows, it can go either way. Regardless, I still think he has some kind of role to play.
Bartolomeo is, well, it's hard to put words on, but it really can go either way for him. While he has great respect for the Strawhats, and loves them for the bottom of his heart, I still believe he won't ditch his own crew. He seems to see Luffy as some kind of role model, and if he were to take Luffy's example, he'd stay with his crew. That's what Luffy would do, and that's what I think he'll do. Then again, his fanboyism could take over his judgement any time, so there's nothing more to do than to speculate.
Personally I think Sugar's gonna be the ninth member, or tenth member if you include Luffy.