Marvel's decision not to use progenital Doctor Strange villain Nightmare in the MCU ultimately had a huge ripple effect, leading to Scarlet Witch's controversial role in Multiverse of Madness . Nightmare was reportedly considered as the antagonist for both Doctor Strange films, but didn't make the cut, something both fans and the film's creators feel was a misstep.
Speaking with The Direct , the writer of the first Doctor Strange movie, C. Robert Cargill, cited early plans for Nightmare as the villain, which were nixed by Marvel. The same thing subsequently happened with the second film.
Nightmare in Marvel Comics
In retrospect, for Marvel fans who weren't pleased with the Strange sequel, Nightmare as the bad guy would have been a dream come true.
Marvel Kept Nightmare