Marvel movies work in threes. They also work outside of their threes. Some of the better Marvel films have more than one villain in them. Me and Scott have not laid the groundwork for it but what I can say is that I have a feeling that whatever Nightmare is involved with, Baron Mordo, being somebody who considers himself the defender of natural law will have something to do with it.
While Marvel's Phase Four plans have begun to take shape behind the scenes, we know very little about what is actually planned. We expect that a Doctor Strange 2 will happen at some point though Benedict Cumberbatch has stated he doesn't know anything about it. It seems that C. Robert Cargill and his writing partner, Doctor Strange director Scott Derrickson, have a plan in mind for the sequel already, they just need to be told to go make it.
Nightmare is a ruler of the Dream Dimension and one of Doctor Strange's oldest enemies, appearing first way back in 1963. Nightmare is able to torment people through their dreams, drawing psychic energy from them that he can use for his own purposes. Having seen another dimension already in the first Doctor Strange movie, it certainly would make some sense to see that element become more important in a sequel.
Based on C. Robert Cargill's comments about Marvel movies working in threes, it sounds like the plan is for the first sequel to continue to build the Mordo story that began in the first film, potentially with it all paying off in a future third movie. Cargill also said during the Q & A (via Movieweb) that Marvel plans to see Baron Mordo's arc from the comics realized in some form on-screen. Something that Cargill says he expects to happen regardless of what filmmakers make it happen.
Of course, before Stephen Strange can have another solo adventure, he needs to survive Avengers: Infinity War and Avengers 4, which may not be all that easy. We'll see the first part of that journey on April 27.