Marco vs King doesn't make any sense. I get it would be a cool fight, but what is the point?
We already know that whoever fights King is going to win so the outcome wouldn't be interesting. This is true for any fight in which the main characters are involved but we want to see Franky vs Sasaki, for example, to see
how Franky is going to win. What he is gonna come up with to defeat a Tobi Roppo, someone that is certainly widely considered more dangerous than our Iron Man.
We already know Marco is at the very least comparable to King
Let the man be useful elsewhere, like against Big Mom the emperor he, more than anybody else, knows how dangerous can be.
He might fulfil WB desire and save Luffy.
Or go helping the scabbards, who cares