The latest rumors quote an unnamed source who explains that Rocksteady were very impressed with the multiplayer portion of Red Dead Redemption, specifically the free roam aspect, and that they’re looking to emulate that for the highly anticipated sequel to 2009's Batman: Arkham Asylum.
"Many people were petrified when they heard of the multiplayer, imagining some tacked on silly arena fighter, the development of which dwarfed our efforts on single player. They couldn't be more wrong."
"...with all dedicated servers we have, you do not have to worry about being in a world with 'Other Batmen.' You will be the only Batman, your buddy the only Joker. Pick who ever you want out of 12 characters, and you will enter a lobby that doesn't already have that character. You may also do private matches. And all this can be done with either 1-12 players, so you can do Free Roam alone, or in Co-Op, or multiplayer."
Almost every building will be fully explorable, and the there will be plenty of citizens to save or harass, depending on whatever floats your boat. Also, the game will feature a co-op campaign in addition to multiplayer.
Batman: Arkham City is set to be released sometime during Fall, 2011!