Mister Rasputin whose real name is Pavel Plotnick is a supervillain skilled in sorcery fought by Dr. Strange in the Marvel comics
The fact that Mads Mikkelson will be playing Mister Rasputin as the villain for the upcoming Dr Strange film this year in the fall is because his character is said to be a sorcerer who breaks off into his own sect from the Ancient One and believes that the Ancient One is just protecting her own power base and that the world may be better off if we were to allow some of these other things through.
And since Chiwetel Ejiofor's Baron Mordo isn't a villain in the upcoming movie like he usually is in the comics, it is doubtful that Dormammu would show up to oppose Strange right away even if the movie does feature the Dark Dimension.
For now we wait and within the next few months Marvel will reveal the villain that Mads Mikkelson is playing soon enough.