This clearly set him up to be a major adversary for Stephen Strange down the line (and thus following in the footsteps of his comic book counterpart), so if Nightmare were to be introduced in Doctor Strange 2, Derrickson and the other creative minds involved would need to do a careful balancing act between the two villains.
Right now, Doctor Strange ranks at 90% among top critics on Rotten Tomatoes, and it’s brought in nearly $340 million worldwide.
However, don’t forget that Doctor Strange ended with Mordo turning his back on the sorcerers and beginning his mission to neutralize other sorcerers.
With a performance like that, it’s hard to imagine that Marvel is already planning Doctor Strange 2.