Ultimate Marvel vs. Capcom 3, Capcom explains the often obscure sources for said costumes. The "Viewtiful Strange" pack that came out last Tuesday, for instance, contains a Doctor Strange variant from a 1960s comic-book subplot, among other esoteric digs.
There is however one costume without an explanation, one that leaves our interests sufficiently piqued. Nova's new threads (as seen above) legally cannot be expounded upon, according to Capcom Unity, which means that the inspiration for this costume must come from a Marvel production that is currently unannounced. We know that Marvel will be making some sort of announcement during the SXSW Interactive festival in Austin next month, and while we're not willing to say that Nova will be getting his own game or anything, it's not beyond impossible for the two events to be related.