I love discussions like this.
For Nightwing, I'd love to see him steal the following bad guys:
Riot. A D-list Superman foe from the 90's. He's a scrawny, crazy little guy with duplication powers. He's just "Arkham" enough to feel (psychologically) fitting for a Nightwing rogue, and his powers provide a unique challenge.
Atomic Skull. Another Superman villain who has never made much of himself. He's a huge guy with a green flaming skull for a head and radiation powers. He always gets overlooked by the Super books but there have been a few moments where an interesting character has peeked through. And he's so, so, soooo much more powerful than Nightwing, I think it'd make for a great battle. Everyone loves the underdog, and it'd be nice to see that Bruce Wayne isn't the only street level vigilante capable of punching above his weight class.
Roxy Rocket. From the 90's Batman cartoons. She's an adrenaline junkie who rides a giant rocket and steals stuff for fun. Sounds like the perfect femme fatale for our boy Grayson. And to top it off, Roxy's a red head.
Professor Pyg. He was created to be a Dick Grayson villain....but not a Nightwing one. Still, the dude is weird and creepy and runs a weird and creepy circus. He was built to bounce off Dick's character, why let him remain a Batman foe now that Dick isn't Batman?
HIVE. I gotta be honest, these guys have never impressed me. But there's potential in DC's own version of AIM and I do love me some super agent/spy Dick Grayson action.