One thing that holds me back from thinking it will be a massive big chapter is that it's going to be smack in the middle of a volume, and Oda usually likes the big moment to be at the end.
Like if we look at Chapter 500, sure Ray showing up was big, but the biggest moment of the volume came two chapters later to finish it off, when Luffy punched Charloss. That was the actual climax of the arc.
If there's something he has in mind he might try and stretch or rush things to get it there, but the way the pacing seems to be going it doesn't really seem likely. Luffy's pretty much right in front of Kaido right now and the war has begun, it's hard to think that Oda can delay something big between the two for thirteen chapters.
A mid arc interlude chapter (all of which have been hype as fuck) or a beginning/ending of an arc seems the only thing that would really fit.