You all know the way these kinds of news posts go by now, so I'll make sure to keep this short and sweet. Most people are pretty familiar with Viewtiful Joe, who has made a few other cameos before this. He also has apparently been given a new voice actor, of whom doesn't sound quite right to me.
Dormammu however, is a different story. The long story is that he's a demon with some sweet powers who fights Doctor Strange a lot. The short one: he's awesome. It just goes to show how much thought is being put into the characters chosen; there is a lot of variety across the board so far and we're not even done seeing them all. In the meantime, enjoy their respective entry trailers.