Killer Moth
Granted, this is a silly choice. Yet, it’s the most comic book accurate of the lot. Babs’ first appearance in Detective Comics #359 involved her saving Bruce Wayne from being kidnapped by Killer Moth, so the loop would be closed if it happened in her first solo outing as well.
Unfortunately, Killer Moth is only killer in name. He’s an incredibly lame villain, and I doubt he’d translate well to the big screen. Mind you, there’s always the possibility that the studio swaps him out for Firefly, or combines the two as a new character, because they do look similar. Frankly, the former and latter both make a lot more sense than a man dressed as a giant moth. Honestly, does that strike fear in you, or laughter?
While Killer Moth is the least likely of options, it’s possible that there could be a nod to the character in some form or shape in the film. Sure, he’s about as cool as Condiment King, but he’s still a vital part of Batgirl’s formative years.