The safe bet would be advanced Coc or an improved armament..but Oda might actually do something else like..
ADVANCED OBSERVATION HAKI
Hear me out, King and Katakuri parallel. Appearance, personality, speedy yet powerful.
Zoro's hasn't tagged King that much. He may need a faster attack or better observation to hit King.
We all thought that Zoro would get his observation powerup vs Shiryu but maybe it's here. Mind you, it's very unlikely though.
Whatever the powerup will be, I expect that we'll finally get another Mihawk flashback during this battle. I'm more interesting in the backstory behind how Zoro lost his eye. Was he trying to learn a technique and it cost him an eye? Was he fighting Mihawk? Or perhaps is his eye still usable?