Depends on whether it was a close fight or not. If Alvarez loses by KO take an easier fight or two to rebuild confidence, and then get a rematch with an older GGG. If GGG loses by KO he'll either: - retire - try to get the rematch - continue to fight other MW contenders and perhaps eventually get a rematch - move up to 168 lb and see whether he fares better there.
I'm wrong sorry, Canelo is not challenging for the WBC but apparently it will become vacant if GGG loses. I mistakenly thought GGG would get to keep it.