Big Mom is going down next. The question is simply when. She's being 2v1'ed, it's entirely different from Luffy winning alone against the arc antagonist.
What I "can" see is this just being dragged out for as long as possible. You tie up all other plotlines, give more chapters of them fighting, give Luffy vs Kaido more chapters of fighting, give them their Rocks flashback, then in a 1, 2 Punch with consecutive chapters, take them both out.