2. Nightmare
Nightmare is a powerful supernatural being who has long been a troublesome foe for Doctor Strange in the comic books. He is a terrifying character who is akin to being Marvel's version of Freddy Krueger, by virtue of the fact that he is the evil ruler of a "dream dimension", where tormented humans are brought during their sleep. Indeed, Doctor Strange has encountered him so often that he routinely casts a protective spell on himself before going to sleep. Nightmare roams his nightmarish realm on his demonic black horned horse named Dreamstalker. He appears as a chalk-white man with wild green hair, a green bodysuit and a ragged cape, and draws power from the psychic energies of the subconscious minds of dreaming beings. He occasionally singles out souls for special tortures, sometimes trapping their dream-selves in his realm. He has even sought to absorb the entire waking world into his own realm - though never with any lasting success. Aside from being fundamentally evil, he is also an essential character for humanity's well-being and vice versa. Nightmare is dependent on the human race's need to dream. Without this ability, Nightmare would cease to exist, but humanity would go insane. At one point in the comic books, Doctor Strange and Nightmare had to join forces to prevent that from happening (not that we're in any way suggesting that should be used as a movie concept, because it really shouldn't). Nightmare's appearance could be used to explain dreams and flashbacks that characters like Bruce Banner, Tony Stark and Captain America have already been show to have on-screen in previous Marvel cinematic universe films and he could be shown to have fed on those very scenes from past movies, which would be interesting. He would represent a very unique concept as far as comic book movies go and would really bring a horror-like feel to the Marvel cinematic universe. It could make for a very cool movie.