I don't think Zoro gets to have a showdown with one the Admirals. I think he will be dueling Mihawk for the title of World's Strongest Swordsman.
imo Akainu will fall to Sabo, fulfilling his duty as the remaining elder brother.
Fujitora will, imo, be elimated or switch sides by then.
Sanji vs Kizaru seems obvious. I think Yamato will be the one that faces Greenbull, which makes sense to me considering what one represents as a devotee to Oden, and the other to the WG that threatened Wano.
After all said is done, Zoro will slander Sanji and downplay his achievements in relation to his own (defeating Mihawk and claiming the title).