Unicron would be perfect. A big planet that destroys everything on its path and heading to earth. The twins on a surveillance post in space that get sucked up and return at the end of the movie. Autobots and decepticons fighting an epic battle on earth but then have to work together with humans to do everything to hold him of. And with the movie ending in a temporary peace. And in the meanwhile unicron sends scouts to earth to begin destroying things and places. In the epic battle on earth megatron could be destroyed and become unicrons slave(as galvatron) to prepare his arrival. And why not just call it transformers 3.