Apparently we might get a Doctor Doom film. If it follows the comic book origin, it might actually be good. I say this despite doubting the average movie-goer cares about Doctor Doom at all and yet has had him crammed down his or her throat three times already.
What about Thanos? Would the early life of Thanos make for a film you'd like to see?
How about Kang the Conqueror?
My premise is - the villain's childhood has to matter. Some guy who was hit by gamma rays and suddenly became a villain (like the Leader or the Abomination) couldn't really sustain a full-length film solo. The villain's antagonist (for example, the Hulk) would have to be the star.
The Mandarin would be great but Marvel currently won't do it. Nevertheless, his childhood matters, so I'll put him on the list.
The Kingpin? Nah, if Marvel really wanted to do it, they would simply dedicate a couple episodes of the Daredevil Netflix program to it.
Any other ideas? Do any of the above sound good to you?
For me, what people fail to realize is that villains' roads to villainy aren't the interesting stories. The story of how Vader became Vader is inherently not interesting. This is the main reason the Prequels sucked.
That said, I wouldn't mind seeing a Cold War Nazi hunt starring Our Lord Magneto. I don't consider him a villain though.