We've already received confirmation you'll be assuming the role of Robin in all challenge maps and Catwoman during certain parts of Arkham City's story, but as of now, the only villain that's be confirmed for Arkham City is Two-Face (and, of course, The Joker.) A new trailer that's been released by WBIE has officially unveiled two more of Batman's well known villains: Penguin and Solomon Grundy.
The video that was released shows the penguin using his famous gun umbrella and then taunting Batman. The focus of the video is on Batman taking down a number of thugs Penguin has ordered to take him down. After showing some devestating hand-to-hand combat, Penguin destroys the ground Batman is standing on to reveal a nice little surprise is waiting for him down below: Solomon Grundy.