None of their sources can agree on what kind of role Ejiofor would take on if he were to accept one, though it seems unlikely that it would be a villainous character, like he's had in Serenity. The report suggests that the most likely character would be that of Baron Mordo, a sorcerer's pupil whose status is knocked down a peg when Strange arrives.
DOCTOR STRANGE: Marvel May Be Talking To Chiwetel Ejiofor For Major Role