He's been kicking around without his crew in the island who knows if they're alive or dead and his alliance with caesar seems really sketchy. He might be persuaded to become a nakama if his crew got wiped out and would be an awesome addition to the strawhats 3 supernovas in the one ship is pirate king material and luffy seems really grateful to him for saving his life and might ask him to join like when luffy asked jimbei to be his nakama after saving his life. It might depend on whether Law follows caesar's orders to kill the strawhats or defend them.
Doubtful. I see him as more of an ally than anything else.
Luffy's SH crew will likely remain small, but his allies will be large in number, since he's a particularly life altering type of dumb guy.
I see Law becoming like Crocodile in a way. With a sense of respect. If the 4 current Yonkou are destroyed by the end, and replaced with 4 new, I could see Law being 1 of them. His abilities are extremely powerful, particularly against paramecia type users without an adequate level of haki.
I'm pretty sure he's going to Betray Luffy at some point, and Luffy won't completely understand it and they'll turn out to be actual friends when everything is said and done. That's my guess anyways.
He wants to become Pirate King. Either he ditches his dream, and that would make him unfit to become a Strawhat Pirate, or he keeps pursuing it and that would also make him unfit to serve under Luffy.
It's just not happening.
Sometimes, the best answer is a more interesting question.
Sanji has pretty much ditched his dream of seeing all blue he hasn't mentioned it in years.
They just haven't came across it. For all we know, All Blue could be within or near Raftel. That said, he was willing to give up his dream (as was Zoro) to save Luffy against Bartholomew Kuma on Thriller Bark...
So I doubt he'd leave the SH crew to remain in the All Blue if Luffy isn't yet Pirate King. In fact, I don't think he'd actually leave the SH crew unless Luffy disbanded the crew or died. The latter would be more likely of the 2, in my opinion, and if that were to occur it'd be shown in a form of One Piece epilogue. For that matter, I doubt Luffy is going to die in the story. I think it'll end before that point in his life.
Doesn't mean Sanji can't use the resources of All Blue to assist his crew though. For all we know, he could be Luffy's crewmember while serving Luffy's many allies good food.
It's lame that the only mentions of the All Blue have been in jokes where Sanji seems to care about women more than it...I'm not gonna get into a whole "Sanji's lame now" thing, but I liked the moment in the Baratie Arc where he tells Luffy about it, and Zeff notes how excited he is. I think when he finally finds the All Blue it'll be a great moment in the series, and I've imagined that moment several times. And I wish Oda would have him reference that every once in awhile so we know he still cares. Yeah, a lot of the other SHs don't talk about their dreams much either but he know they still care. No way Sanji's leaving the crew just because he finds the All Blue though.
I guess I'll talk about the ending too...I think Luffy's gonna die. Not in his fight with the final enemy, and not soon after, but before the very end he will. My prediction for the end of One Piece (right now) is that Luffy becomes pirate king, a number of years pass, and he dies, but before he does he passes the Straw Hat onto Makino's baby from the Decks of the World cover story. The young man starts out on a journey, meets an intriguing stranger and gleefully asks them to join his crew. The end. The dream goes on. Obviously there's not evidence for much of this but I think it'd be pretty cool. I also don't think the final enemy will be Blackbeard but that's another story.
______________________________________ The question is, who wouldn't?
I like these predictions, but I also really think BB will be the final villain. He has the strength t back it up (yonkou and several DF), and he's basically Luffy gone bad. How much more antagonistic can he get?
Sometimes, the best answer is a more interesting question.