The MCU is hardly the only film that lacks in developing the antagonist in their films. Plenty of films struggle with this because the film is busy developing the protagonists.
This probably explains why so many of the MCU TV shows have really well developed villains, because they have the time to do both.
While obviously we'd all like Doctor Doom to take on The Avengers, I don't think having Doom is a get-out-of-jail card. Hell, Fox has screwed up Doom twice already.