Develop Them More

The Marvel Cinematic Universe’s villains have suffered terribly, as a general rule, from a lack of character development – and that’s thanks in no small part to the aforementioned point about them being introduced and killed off within a single movie.
We never really properly got to know Malekith, Whiplash, Ronan, or Abomination. They were just thrown in front of us, we were told they were “bad”, and they were defeated without being developed in any meaningful way.
In future, villains should be given more detailed and interesting arcs, so that we know more about their motives, their origins, and their personalities – as we do with the heroes of the Marvel Cinematic Universe.
Now let’s move on to a bold idea that could take influence from DC.
