They have to go back to the rooftop because one of the supernovas will die against a Yonko. My money is on Killer.
But Oda can give us the most cliche death in One Piece, Law's death against Kaido.
If Zoro and Law don't return to the rooftop, then I expect them to fight King and Queen. Even nerfed Law and heavily damaged Zoro can beat a calamity. We can see it occuring but what would happen to X-Drake, Apoo, Cuck, Marco, Yamato, and the scabbards? Who are they going to fight?
That's why Zoro and Law going back to the rooftop is a big possibility.