I agree, I think Koby is the one. At worst Fujitora, but Imho it's not very likely.
And yes, I don't know what Oda is doing to do, but some move on Kamabakka is definitely possible.
Imho, we won't see a classic 1vs1 showdown between SH and CG.
And also, Boa is practically a super groupie of Luffy: she will never fight against him or lay a hand on his precious nakamas. Even if she has escaped and even if she will join CG (something against his character, if I have to say), she most definitely won't fight Luffy under any circumstances.