Airwave's name just seems tailor-made as an equal-opposite character for Specturm (or whatever they're calling Monica these days).
Mister Terrific implies skills that would be a perfect nemesis for (depending on how characterized) either Captain America or Mister Fantastic.
Doctor Midnight sounds like a foil for either Brother Voodoo or Dr. Strange.
The Spectre could be used for anything from a mystic character to a Batman-esque vigilante.
Baron Winters: A mad scientist or a rival to Count Nefaria and Madam Masque.
Off topic, but I always thought that if you dumped Deathstroke into the DCU, he and Captain America would be almost inevitably drawn into an archnemesis relationship.