"We see glimpses of something called the Dark Dimension," Feige explained. "If you were to open a 'Doctor Strange' comic drawn by Steve Ditko, you would see the Dark Dimension is, in fact, very colorful in an extremely psychedelic way. Those are the things we're not shying away from."
It's worth pointing out that Strange's archenemy, Dormammu, resides in the Dark Dimension with his sister Umar. Umar has a daughter named Clea, who has been both a student and lover of Strange and sporadically rules the Dark Dimension herself. With the confirmation of the Dark Dimension's inclusion in "Doctor Strange," it's possible that all three of these characters could appear in the film.
He also shared that the film "couldn't be any more different than any of the films that have come before it... There are more things that lurk in shadows in this movie, perhaps, than our other movies."