Ziggler already getting his rematch on Smackdown doesn't mean he can't very well be given another one. This is WWE, it's not like their rules have any absolute value to them. Guys have gotten more than one rematch before when they shouldn't have, and guys have also not been given a rematch at all when they should have been, etc. If they decide to continue the rivalry between Ziggler and Harper, they can easily just give Ziggler another shot for no other reason than because he's feuding with Harper.
Unless Lesnar shows up and defends the title against Cena, I don't know who else Cena can wrestle except for Seth Rollins. Big Shoe WAS an obvious candidate, but he doesn't seem to have any issues with him over what happened at Survivor Series. And in that case, I don't know who else Dolph can compete against except for another match with Luke Harper. I though Harper might defend the strap against Rowan, but that match seems out now that Rowan and Big Show have some tension with each other.