Dormammu

The most powerful enemy of Doctor Strange, Dormammu is the ruler of the Dark Dimension. Mads Mikkelsen is the perfect actor to bring this villain to life, although we won't actually see him in the flesh "“ a giant demon, Dormammu can only be brought on the silver screens via motion capture.

Of course, it may be a bit early to introduce this powerful Doctor Strange enemy (especially since there are rumors that he will become the next threat of the MCU, after the defeat of Thanos). Nonetheless, Mads Mikkelsen can make just a brief cameo, as an ally of Baron Mordo.

To be noted, he has made several other alliances in the comics, most notably with Loki.