to be honest i think Zoro will have to fight King, if Marco was supposed to take down King we would have seen their bounties already. King's bounty was not revealed yet for a lone reason and that is that his final opponent is not there yet.
Why do people want to see Marco taking out so badly?
Do you think Oda brought Marco just to be taken down without doing anything or defeating a strong opponent?
He is obviously there to fight, and King is the perfect opponent for him and fits his caliber.
Zoro vs King may happen but I doubt it because of Marco. Where do we put him then?
Also, whoever fights King needs to fly, and Zoro can't fly.