I’d love to see Gotham’s first super-villainous gang war, entirely inspired by the Joker’s psychotic shenanigans. I see them as two small-time crooks, languishing in dank corners of the Gotham underworld when they see what the Joker accomplished with willpower and a new face and two super-villains are born, filling the void left from Dark Knight.
Someone on RPG.net posted a theory for the Riddler to be a Gotham cop who is charged with bringing down Batman. That is pretty keen too. Riddler as a federal agent tasked with bringing down Gotham’s vigilante would be neat.
I think a Catwoman could fit in there too, as dense as Dark Knight was, but that would be stretching it.