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.
Self-contained stories part of a larger story is where its at. Not this writing for the trade nonsense. If you're going to write for the trade, don't release it in individual issues. Just release the trade.