in my opinion, Zoro and Sanji should fight the admirals (Fujitora and Kizaru) and also the goroseis (bald and blond). This in the final conflict against the world government.
You know, I don't think anyone other than Luffy will fight Akainu, he needs that rematch, so when that happens, Sanji and Zoro will fight the admirals. And in the final war, Luffy vs Im and Zoro and Sanji vs Gorosei.