@Seafarer33:
I agree on that, but who will fight the Calamities then ? Someone has to, or they will interfere on the roof. Marco is down there, alright, but 3 is a bit much even for him (I assume Jack will show up again). Who then ? Jimbe and Drake could be up to the task but have their hands full right now, Franky as well plus a Calamity is maybe starting to get out of his league. Chopper and Brook ? Nami and Usopp ? Sanji ? Not to mention the rooftop fight might require reinforcements. Things are looking good right now, yet the Straw Hat alliance still looks understaffed unless there's another twist incoming.
By process of elimination, Sanji pretty much has to fight one. The only other alternative at this point is to gang up on Tobi Roppo with another Strawhat, which would feel quite lopsided given his raid suit performance against Page One. If there was a Tobi Roppo remaining he could fight by himself, that'd be one thing, but that option no longer exists.
Queen is the most likely given the Judge connection/awareness, but King fits very well also.
Sanji aside, it's also possible that Jinbe will swap opponents. Depends if you think it's a disservice to Jinbe's strength for him to not test his mettle against a Calamity. That said, we don't know much about Who's Who's strength admittedly. He did want to challenge the Calamities after all.
And then like you said, we have Marco and X Drake to face off against whoever is left.
–- Update From New Post Merge ---
@Monquito:
Nah, pretty sure the roof is just goin to collapse, separating everyone.
Doesn't matter what shenanigans occur, I am confident that those 5 characters on the roof are only fighting the Yonko. Oda purposely went out of his way to avoid having them fight the Calamities, that won't suddenly change. At best it will be another Law in Dressrosa situation, where they get beaten up, then recover and watch Luffy finish off Kaido.