Doctor Strange was a solid hit for Marvel Studios and since we know that Stephen Strange isn't going anywhere in the MCU, there will almost certainly be a Doctor Strange 2 at some point down the road. Director Scott Derrickson certainly seems to have a few ideas about where the story might go and what new characters might appear. First, Derrickson appears to imply that one character's appearance might be inevitable and possibly even intentionally set up in the first movie.
Brother Voodoo is the name given to Jericho Drumm, who eventually becomes Doctor Voodoo later in the comics after actually taking over from Doctor Strange as the Sorcerer Supreme for a time. Jericho has a twin brother named Daniel, whom we've already met. Daniel Drumm was the man in charge of the New York Sanctum when Stephen Strange arrived there after escaping the attack in Tibet. When Kaecilius attacks the New York Sanctum he takes down Daniel first. Derrickson seems to be implying that Daniel's death could cause Jericho Drumm to react in some way, potentially making him part of a future sequel.
It would certainly be easy enough to bring Jericho Drumm into the fold. Since he and Daniel are twins, you can bring Mark Anthony Brighton back to play the role of his own brother.
Doctor Voodoo isn't the only person that Scott Derrickson sees a potential place for in a future Doctor Strange film. He's also got his eyes on a villain that he'd like to bring into the series as well as one of the Sorcerer Supreme's most popular allies.
Nightmare is the ruler of the "dream dimension" in Marvel Comics. While the first Doctor Strange introduced concepts of magic and alternate dimensions to the MCU, it's possible that Scott Derrickson feels that the dream dimension, and a character that feeds off of human dreams in order to survive, may be a bit more difficult to bring into the current movie universe. Having said that, it would truly be awesome if it could be done.
Finally, Scott Derrickson mentions Clea, a character who becomes a significant love interest for Doctor Strange in the comics. As the director says, you can't really leave her out of any proper telling of the Doctor's story, so there's a good chance we'll see her at some point.
What other characters would you like to see in a future Doctor Strange sequel? Let us know in the comments.