Warning: spoilers for Doctor Strange are ahead!
Doctor Strange saw the eponymous Marvel hero battling two primary villains after months of studying the mystic arts: Kaecilius, who led a sect of followers equally dissatisfied with The Ancient One, and Dormammu, ruler of the Dark Dimension. However, these baddies weren't always lined up to be the movie's principal antagonists. In one of Doctor Strange's earlier drafts, director Scott Derrickson wanted Stephen Strange to throw down against Nightmare.
Derrickson revealed to Empire that he originally wanted Doctor Strange to fight Nightmare, the ruler of the Dream Dimension who was Strange's first adversary when the sorcerer debuted in 1963's Strange Tales #110. However, Marvel Studios president Kevin Feige argued against Nightmare's inclusion to avoid overcomplicating the story. Derrickson explained:
Kevin [Feige] made a very cogent case. The trouble with starting with Nightmare is getting across the idea of the Dream Dimension as another dimension. The movie was challenging enough. It's already an exposition-heavy movie... Dormammu made the most sense. And he is the most present villain in the comics.
Along with fully showing off magic in the MCU, Doctor Strange introduced several alternate dimensions that Earth's sorcerers can travel to using their abilities, like the Astral Dimension, the Mirror Dimension and Dark Dimension. In the Marvel comics continuity, the Dream Dimension is one of these alternate realities, where Nightmare brings tormented humans while they're sleeping. It sounds like Kevin Feige didn't want to confuse casual moviegoers by having dreams themselves be an alternate dimension, which is why Scott Derrickson and the others opted to use Dormammu and the Dark Dimension (which is conceptually less complicated) instead. Plus, to be fair, Dormammu is the more famous member of Doctor Strange's rogues gallery.
Despite Nightmare being excluded, Scott Derrickson is hopeful that the entity can be used in a sequel, which seems like a likely endeavor. Right now, Doctor Strange ranks at 90% among top critics on Rotten Tomatoes, and it's brought in nearly $340 million worldwide. With a performance like that, it's hard to imagine that Marvel is already planning Doctor Strange 2. However, don't forget that Doctor Strange ended with Mordo turning his back on the sorcerers and beginning his mission to neutralize other sorcerers. 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.