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?
Diabolical, Sadistic, Twisted Villains (for most people) are hard to relate to, but everyone enjoys a caper movie. So a caper told entirely from the villains viewpoint, in which some hero (or heroes) is an obstacle to the goal could be fun.
Having not seen it, I may have just inadvertently described the plot to Suicide Squad.