Here's a couple thoughts

What about Azrael. I crazy ass religious serial-killer / vigalante. Could be fun, with that flaming sword and all. At first it seems he's a lone psycho, but it turns out his religious order is infiltrating all aspects of Gotham City.

How about multi-villains, but just the lightly gimmiccked organized crime themed ones. With the regular criminals all taken out during "The Dark Knight" more freaks come in and start a massive gangland war. Black Mask. Penguin. Tally-Man, Kobra, El Penitente (he's new). Scarface.