Re: What villains are left for a 4th film?
Their's the obvious choice of Unicron, though with everyone dead I doubt it. If he hadn't been poorly used and killed I would say make Shockwave the PROPER villain he should have been/deserved to be. Or, if all else fails...make one up.
One may have a blazing hearth in one's soul, and yet no one ever comes to sit by it.
Vincent van Gogh