I agree with this. And when you factor in other rogues like Lady Vic, Blockbuster, Torque, Brutale, etc., from the Dixon era on top of Raptor, Deathwing, Prankster (already nabbed from Metropolis) and the Judge as well as Pyg, Flamingo, and the other DickBats villains, Nightwing's got a damn solid foundation of rogues. They just need to be used and developed. The problem, as always, is consistency. The bad guys Dick has are fine on a conceptual level, but writers rarely use them.
But if you're gonna steal other villains to pad things out (which I totally support) then I can think of several villains to take from the different corners of the DCU that Dick is close to.
Roxy Rocket. If you can find a better bad girl to fill in the "Catwoman/femme fatale" slot in Dick's rogues gallery, I'll send you five dollars.
Roxy is a crazy, high-flying adrenaline junkie who steals stuff mostly just because its fun to make the cops chase her. She's shown up so rarely she's basically a blank slate for Dick's writers to develop. And she's a redhead.
The Bomb Squad. The team of Electrocutioner and Plastique make for a really fun Z-list villain team, and Dick needs villains he can take on without it having to be a huge, issue-long battle. These two are ridiculous, they know it, and they soldier on anyway. Who could ask for more?
HIVE. A international terror group combined with super scientists, HIVE is basically the DC version of AIM and they've been a thorn in Dick's side since his days with the Titans. They work well for espionage/spy/thriller stories or for more straight-laced superhero narratives about giant evil robots and plots of world domination. And the various versions of Queen Bee, from the creepy alien hive-mind slaver to the mind controlling ruler of Biyala offers a ton of interesting ways to take the organization.
Red X. I never watched the old Titans cartoon, but I feel like you can never go wrong with a mysterious villain with a cool mask who has abilities to equal the hero and rocks the unknown, mysterious origin.
He could be Dick's Deathstroke if handled carefully.
Atomic Skull. It's always fun to see the hero tackle a villain who utterly overpowers him, and the Skull can survive a couple rounds with Superman, making any conflict a major (entertaining) struggle for Dick. And like Roxy Rocket, the Skull is largely undeveloped and unused by the Man of Tomorrow. Becoming a Nightwing foe opens up lots of room for the Skull to become a major player, rather than an also-ran, while providing Dick with a physical threat beyond anything he usually faces without a team behind him.
Riot. Anyone who remembers this D-list Superman villain probably wishes they didn't, because he was just the worst. But an insane nihilist who's basically a city-wide mob all on his own (he was a duplicator, for those who dont know) has potential as a more street level threat and could pose a fun, interesting challenge to Nightwing.