6. Vincent Cassel - Baron Mordo
With Doctor Strange's universe likely to be populated by demons and inter-dimensional creatures (such is the nature of the character after all), his mini-franchise will require a more human-like villainous element to ground the rest of the foes: a Loki-type to balance the supernatural elements - and Baron Mordo fits the bill perfectly. Mordo would also fit Marvel's tendency to employ character mirrors - Iron Man had Iron Monger, Tony Stark has had both Justin Hammer and Aldrich Killian, Thor has Loki, Captain America has the Winter Soldier and the Hulk had the Abomination - since they seem to be under the impression (mostly well-founded) that perverse similarities make for for enduring villains. Mordo fits in with that pattern, since he is basically the antithesis of Strange, trained by the same force (the Ancient One), with Strange basically established by his master in order to nullify Mordo's threat to him. It would make sense then to cast a charming actor with a dark side who could well work as Doctor Strange himself, which is firmly where Vincent Cassel fits. He has that intangible otherness that lends well to villainous roles (much like Mads Mikkelsen - another strong contender for the role) and his dramatic chops would help pull the performance away from pantomime territory.
Who It Definitely Shouldn't Be: The same actor who plays Doctor Strange. Even though similarity is a big winner for Marvel villains, introducing am evil twin gimmick would be awful.