With Mads Mikkelsen playing Kaecilius and Chiwetel Ejiofor playing Baron Karl Mordo, all the early fan favorites to play Dormammu have assumed other roles. Unless he's being saved for the sequel, the film needs to cast someone to play the master of the Dark Dimension. Could that be who Benjamin Bratt is playing? Marvel has not made any announcements to alert the public that Bratt is in the film so either his role is minor or he's playing a character that they want to leave a surprise until opening night.
Who do you think Bratt (52) could be playing? Recently, he's been doing a lot of voice over work (Justice League: Gods and Monsters, Despicable Me 2, Cloudy with a Chance of Meatballs 2) which is pretty much all the role of Dormammu would require. Of course, this is only speculation (and maybe wishful thinking) so until we hear something official from Marvel or the Hollywood trades consider Bratt's role a mystery.