9. Mister Sinister
Mister Sinister was originally going to be an 11 year old child who lived at Cyclops' orphanage. The name, the black lipstick and eye liner, the white face paint, the red eyes and shredded cape are all the design of a child in order to scare other children. To any adult however, he is absurd. Born in the 19th century as Nathaniel Essex, Mister Sinister has an unhealthy fascination with genetic mutation. Through unethical experimentation he modified his own body to make himself effectively immortal. The name Sinister was given to him by his own wife in her dying words. He developed an even unhealthier obsession with the particular genetics of Scott Summers (Cyclops) and Jean Grey. This obsession led him to create a clone of Jean Grey for the purpose of making a super mutant child with Cyclops. The clone did have a child with Cyclops, and then in true comic book style went insane and opened up a gateway between dimensions to bring about a literal hell on earth. Mister Sinister is one very creepy and twisted stalker.
How could he possibly work in film? Visually, Mister Sinister would fit pretty easily into the X-Men film universe. There is already a naked blue lady, a furry blue man and a few demon-like mutants. A goth looking mutant like Sinister would seem comparatively normal. Mister Sinister would also give an opportunity to see some aspects of the X-Men universe not yet explored in film. It would introduce the idea that mutants have been around for a very long time, operating in relative secrecy to the world. This would pave the way for another ancient mutant and classic X-Men villain, Apocalypse. Cyclops has been a throwaway character in the films to date. Mister Sinister's obsession with Cyclops would highlight the importance and power of Cyclops, who is a key character in the X-Men universe.