IGN recently caught up with Doctor Strange helmer Scott Derrickson, and while a sequel has yet to be confirmed by Marvel Studios (despite positive reviews, it all comes down to box office), the filmmaker already has some ideas about the direction he'd like to take things in. In fact, he specifically mentioned a villain he thinks would a be a good match for Strange, and it's probably not who you're expecting.
"I really like the character of Nightmare and the concept that the Nightmare Realm is a dimension. [...] That’s early – that’s like the first Strange tale. I think that’s in the introductory episode of Doctor Strange, and I always loved that." The character did indeed appear for the first time alongside the Sorcerer Supreme back in Strange Tales #110, so why didn't he end up making it into the movie?
"We decided not to do that because that’s a bit of a complex idea to try to introduce everything we do introduce and introduce the idea of nightmares themselves as being a dimension," Derrickson revealed. "I hope somewhere down the line we do get to explore that because I think that’s super cool concept." We'll have to wait and see whether or not that happens, but it would be interesting to see Strange battling Nightmare. For now, the hero will next appear in Avengers: Infinity War.