Sticking with DC heroes who've actually gotten solo movies to date.
Superman: Corporate Luther. *cough* I'd be partial to Metallo, too, as Luther's muscle, both for that he's an effective source of anti-Superman muscle, and also can serve a useful narrative role in 'Showing that Luther is the Devil'. Which is to say, when you get to the point in the "mad scientist creates a monster" story where normally, the monster rebels and the mad scientist's hubris is revealed? Corben finds out how absolutely he is enslaved, and that Luther is a genius who mostly *is* as good as his megalomania claims.
Batman: Two broad categories of villains- the super kung fu types, and the monsters. We've gotten plenty of the gangsters and madmen, lets see people like Lady Shiva and Clayface, who would let you explore other facets to Batman for a change.
Wonder Woman: Yeah, I have to agree about Circe. She's a villain whose also a perfect foil, in that she's also an outsider demigoddess in Man's World who has opinions on the place, plus she has a distinct powerset. Honorable mention goes to Giganta, who I'd love to see as essentially a super mook with a habit of crossing paths with Wonder Woman, but where there's no actual antipathy on either side. Giganta's not out for vengeance, isn't a bloodthirsty psycho, she's just a crook with flexible morals and enough power to enjoy punching it out with superheroes.
Shazam: Captain Nazi. Because who doesn't want to see Billy Batson and his adoptive family punching the **** out of bigots? *eg*
