Two-Face: great villain for DD and Punisher. So misused in the DCU.
Parasite: I've said this a thousand times, Parasite would be a great Hulk (and even Thor) villain.
Deadshot: I can see him being a good rogue for Hawkeye or Winter Soldier
Zoom (Hunter Zolomon): "Ironman, I want to make you a better hero"
Black Adam: Norse vs Egyptian. Thunder vs. lightning. Thor vs. Teth.
Circe: Foil for Wanda? Yep. Foil for Doc Strange? Yep. Foil for the Defenders, regular or Fearless? Yep.
Gorilla Grodd: the FF love looking for new civilizations, be it on earth or elsewhere. Gorilla kingdom would be perfect for the FF to explore, and Grodd bring a perfect foil for Reed.
The Secret Society
The Ghost: a perfect foil for Batman and Bruce Wayne. Bats needs villains that arent psychotic, but someone after his secrets.
Bullseye: Nightwing is a natural acrobat, and incorporates that into his fighting style. Seems like a perfect challenge for someone who never misses.
Executioner: if you don't think a Skurge vs Wonder Woman battle wouldn't be epic, you need your head examined.
Baron Blood: Long before Batman was the protector of Gotham, the GL Alan Scott was and he his arch nemisis was most gothic indeed.
Klaw: a Hal Jordan baddie. Can objects created by light defeats objects created by sound?
Kraven the Hunter: who will be hunted? Nightwing, that's who.