You have to question if Oda is going to dedicate time to two YCs fighting however. Im honestly not sure why King's fire would be any different from Big Moms but I guess thats up to Oda.
One Piece is mainly about underdogs taking on people who don't normally lose. While Marco vs King would be fanservice-y and entertaining, I do have to wonder if that is a route Oda would go, precisely because he normally doesn't waste time on these people fighting each other (Kaido vs Big Mom, BM vs Marco, Oden vs Kaido, Roger vs WB, etc).
To me, it's just way more useful and pertinent for a Strawhat or Supernova to fight King. We can say that the rooftop battles are solidified, but until we actually see how every matchup is done anyway, none of these are actually guaranteed. Marco vs King isn't any more set up that anyone else, outside of this fan idea that their parallel of hitting BMs ship is supposed to suggest they fight.
We'll see what happens, but im not confident in a full drawn out battle between Marco and King with a clear victor