What if Unicron is simply used as something Megatron created as a last resort to finally defeat Prime? Megatron is embarrased and humilated at his latest defeat and wants to finally get rid of Prime along with Earth too. The autobots must rise to the task and stop this latest threat, hopefully saving Earth and its people from Unicron while finally stopping Megatron. Maybe we can get Unicron upgrading Megatron into Galvatron (as a sign of desperation) for some epic final battle scenes? Or is that too weak of a story outline to make a move off of?