My prediction is Vegapunk.
He's the only big name that matters IN Wano with CP0 deal with Orochi and this Numbers business and OUTSIDE Wano with SSG and the Warlord Abolition during intermission chapters of acts.
It feels like Wano is leading up to the man and honestly, given that chapter 957 and the oden flashback exist, there is like so little mystery left outside the actual endgame and this man.
Chapter 1000 will be the reveal of Vegapunk. Whether inside Wano it's revealed he was actually kidnapped and brought to Kaido by CP0 or maybe we get him in a Kaido flashback involving Punk hazard. Or Vegapunk is revealed outside wano with SSG and what it actually does.
Either way he's the most likely candidate for 1000 worthy reveal as par the things set up in the story so far.