Zoro will presumably be the one to kill Kaido. Luffy never killed characters advertently, and Oda has been portraying Kaido's words "perfection can only be found with death" probably for a reason in that so specific, and also Zoro's specific declaration himself. And all those experiments Kaido conducted on his own body. Put all elements together.
Luffy declaring "I got this" was presumably a circumstantial matter linked to Zoro's poor physical conditions. He would most likely not have spelled those had Zoro stood substantially healthier.
If Kaido manages to resist Luffy's assault enough time before Zoro returns to the battlefield to check what's occurring (cause he presumably would) and/or Big Mom helps Kaido out, we could most likely see this coming to fruition at long last.
The scenario where Kaido is defeated and only later Zoro still cuts his head off strikes as less likely for me now, but that also has to be accounted.
In my opinion there will be more huge battle a head. Roof battle only decide luffy strong enough to be called next yonko.
Pero2 vs nekomamushi
Chooper vs queen
King vs marco
Orochi vs momonsuke?
If im not wrong orochi got 1 more head
BM vs BP member or flying six
Sanji vs ....
Zoro with hiyori again i think so he can healed up
I don't think Zoro is going back up to the roof, but I also don't think the battle is gonna stay up there for long. Could see Luffy/Kaido coming to a temporarily stalemate due to Onigashima (or at least the dome) collapsing.