5. Mephisto
Mephisto - who has already been depicted, albeit very poorly, on film in the Ghost Rider movies - is a reality-warping, super-strong, unkillable magic user who lords over his own hellish realm. A decent depiction of the character in the Marvel cinematic universe would go a long way towards making up for Peter Fonda and Ciarán Hinds' sorry adaptation in Ghost Rider and Ghost Rider: Spirit of Vengeance - in which he was a pale shadow of the incredibly powerful and monstrous being he is in the comic books. Although Mephisto isn't considered to be an arch-enemy of Doctor Strange in the comic books, they have clashed a number of times before and a truly evil, devil-like character in the Marvel cinematic universe would add a very interesting dynamic. Doctor Strange at his classic, most powerful levels, was actually able to pull Mephisto out of his realm just to grill him for information on one occasion, but a novice live action version of Strange would need to be far more careful, as Mephisto would be too much for him to deal with so nonchalantly. On another occasion, Doctor Strange helped to free the soul of Cynthia von Doom - Doctor Doom's mother - from Mephisto's captivity, so that she could ascend to heaven. Overall, Mephisto would be a mystical threat that could potentially take over the Marvel cinematic universe's Earth and Doctor Strange would be the only heroic character capable of putting together a means of stopping him.