Vixen: To me the ideal Vixen villain would be a rogue with a very versatile power or power set. They wouldn't be cosmic level earth shakers but they wouldn't be street level either. In short, mid tier super villains with very versatile powers, or low high end villains that are only high end in a very specialized skill.
Examples: A mid tier telekinetic, might make for an interesting against Vixen because, most of Vixens attacks are close range attacks, and telekinetics are for the most part, distance fighters.