Doctor Strange: 9 Big Implications For Phase 3 And Beyond
3. Other Threats
There are a few hints at what's to come throughout the movie. Doctor Strange mentions looking out for "other threats", while at various turns Wong and Mordo have stark warnings about looming threats and how the events of this movie will have far-reaching implications.
Had they not dealt with Dormammu so swiftly here, he'd have been the obvious choice, setup as a major background villain and ready to come to the fore in Doctor Strange 2, or something even bigger. That's still a possibility, but it'll require some creativity to get out of what they do with him here.
It could also just be a hint at Thanos: we know he's on his way to Earth, and perhaps Strange will learn of the threat he poses and warn the Avengers (although there's a good chance that role will fall to the Guardians).
Beyond that, it's not so clear. But with Thanos likely to be dealt with in 2018, or 2019 at the latest, Marvel will be looking for their next Big Bad, and Strange's alt-dimensions gives them more possibilities for that.