Originally Posted by
Dred
Axel Walker (Trickster), Evan McCulloch (Mirror Master), Hunter Zolomon (Reverse Flash/Zoom), Curtis Engstrom (Alchemy), Alexander Petrov (Mr. Element, amusingly another Doctor Alchemy "legacy") and the mentioned Owen Mercer (Captain Boomerang) are the 'second gen' ones. Though Evan technically originated in Animal Man as a Morrison creation.
The other Wally original ones I can think of are The Johns Era new generation of Rogues with Double Down, Girder, Blacksmith, Fallout, Murmur, Tar Pit, Plunder, Magenta, Peak-a-Boo, and that weird Rainbow Raider crew which was a mishmash of a bunch of nobodies.
Then you got your WML/Baron era Rogue villains like Kilgore, Icicle, Mota, Blue Trinity (which doubles up with Christina who later becomes Lady Savitar), Speed McGee/Speed Demon, and those three elementals whose names escape me. One shacked up with Wally's dad. Because why not.
Then you got your Waid era villains. Kobra, Savitar, The Combine, Team Turmoil, Cobalt Blue, Grimm, Chillblaine, Replicant, Razer, Folded Man, and Neron (created for an event, but also by Waid and used in Wally's solo).
Then you got your Morrison/Millar era stuff which includes The Fashioneer, The Suit, Black Flash, The Cosmic Gamblers, and the fearsome SEASNAKE.
There's almost certainly a dozen or so one off villains like SEASNAKE who I wouldn't remember. These are the ones I could list off.
I'm also not including one time antagonists who turned friends like John Fox, Fury, Chunk, etc since they're more largely recognized as allies.