It has been revealed that a villain that goes by the name of Calender Man is going to appear in Batman:Arkham City. Whether he is going to play a major role or not is unrevealed. It has also been revealed that his personality and interaction can change depending on the day, time or even date. Set your consoles time a month ahead and you will get a completely different conversation from your last.
As well as Calendar Man. Two-Face, The Riddler, Mr Freeze and of course the Joker are all confirmed to make an appearance in Arkham City. Victor Zsasz is also rumored to be in it. With all these Villains and a lot more to come, Arkham City is sure to be more explosive then the first one.