Weren't they supposed to create a brand new villain just for the game? Did they change their mind on that, or did they just mean new as in "not joker"?
Yeah, here's what wikipedia says:
The game introduces the villain Arkham Knight, a character created specifically for the game by Rocksteady, DC Comics CCO and comic-book writer Geoff Johns, and DC co-publisher and comic artist Jim Lee. The Arkham Knight is described as a militarized version of Batman, with the "A" logo of the Arkham Asylum facility worn as an emblem on his chest.