Re: What villains are left for a 4th film?
Transformers unlike people can always be rebuilt/remodeled that I'm sure Megatron can come back even if they have his head in a floating in a jar filled with energon.
A few of the Marvel Superheroes are somewhat niche characters that aren't extremely versatile, thus they don't have a lot of longevity potential. For example, Namor is a water guy, Silver Surfer is a space guy, Dr. Strange is a magic guy, and so on.