Re: What villains are left for a 4th film?
I never understood the resistance to the Dinobots. Nearly every fan likes them. Yet highmoon had to beg to include them in the fall of cybertron game, and I remember reading once that Bay didn't want them either. On paper robot dino's may sound goofy, but it's been showed repeatedly they can be the badass muscle of the autobots.
Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.