I've liked the idea of Doffy teaming up with Teach. Sort of a mirror to Luffy's and Law's alliance.
Hancock makes the most sense as an ally to Luffy at this point.
I don't know whether Kuma can even fight anymore, but if there's anyway he could. then I'd have it alongside Sabo and the other revo commanders during the final war.
Mihawk vs Zoro is a no brainer, though I wouldn't mind some clash with an admiral. Particularly Ryokugyu.
Weevil, to me, screams future ally. He could turn on Bakkin/decide to do things his own way. Even if he isn't Newgate's real son, I think he still genuinely admires him as his true father, and will have a moment when he thinks "what would dad do?"
I don't know who Buggy might fight. He could go the rest of the series failing upwards and involving himself in things incidentally like he always has.
Who Moriah could fight really depends on if he submits to Teach or not. He was in a very precarious situation last we saw him.
Crocoboy appears to want to become Pirate King again, but I don't think Oda plans a rematch between Luffy and him.