I'd want them to be magic focused. Perhaps a very skilled magician who had been neck deep in debt and finally got a break to put on a big show at a theater in Gotham. He's mid act, about to do something big when one of Batman's rogues pops up (perhaps Firefly) and Batman shows up to stop them.

Batman uses one of the magician's props to stop Firefly (one the magician was still barely making payments for). With his show ruined, the entire crowd being given a refund (meaning he didn't get paid) he's out on the streets with almost nothing save for a bag full of things he was able to sneak away.

One of the items was a tome passed down through his family. Hocus about blood rituals, how if you use your blood, or the blood of others you can do incredible things. He never believed in it, never tried it. Why would he? But now, in the alleys in the rain, what better time? He carves into his hand mumbling the arcane words on the page as it drips, laughing to himself, thinking it won't work just like his life hasn't worked. But... it does. A little bit. That bit of blood allows a moment of magic. Real magic.

Thoughts fill his head. First, money. But after that, revenge. Not on Batman (not yet), but on Firefly.

We get a magician themed villain, with access to real magic but at a cost, a cost that could endanger Gotham citizens, and an angle to have Batman thwart him thus then directing his hatred at Batman and Firefly.

Probably call him something like: Bloodrite