Zoro will fight Kizaru after Luffy clashes with him. With the current trend of these defeats I think another old man is likely to bite the dust as well. This is why I think Luffy's main fight will be against Saturn. This will also open up a spot for a new Gorosei to take Saturn's place.
It's Shanks' turn.
I also have a feeling Zoro's kingdom of hell will recruit Kuzan and Issho, and Zoro will need to prove his strength to do this. I feel like Akainu, Kizaru and Ryokugyu are deadlocked on the Marines side. If my rainbow theory for the admirals is right, then Fujitora needs to be replaced by an Orange Admiral to complete the pattern of R-O-Y-G, with B-I-V on the other side. I don't know who the Indigo Admiral is, but Zoro did fight someone named Dr. Indigo in a movie. Zoro has also had meaningful clashes with both Issho and Kuzan. If Luffy wants to be King to defeat Imu he needs a force equal to Akainu and the Admirals. This is why Zoro needs three Admirals under him. Of course the protagonists get the blind and lazy Admirals.