Re: What villains are left for a 4th film?
Like I was saying before, you can just spend a movie or two building up to Unicron. He doesn't have to appear for awhile. Just be a voice in the shadows directing his vanguard (who would be the villains the autobots fight in movie 4) to his next meal, Earth. End of movie 4 they return defeated with the corpse of Megatron. Movie 5 Megs returns as Galvatron, heralding Unicron's return. Unicron doesn't actually appear until the end of movie 5, or in 6. They can introduce him to set up the story without blowing the budget until the huge fight culminates in 6.
Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.