From the IMDB FAQ...
"The villain(s) are unknown at this time, but it has been stated that the Joker will not return. With Ra's Al Ghul and Two-Face both dead, this leaves Scarecrow the only potential returning villain.
As of now, it's been confirmed that Mr. Freeze, the Riddler, and the Penguin will not be included. Popular fan belief is that Catwoman or Black Mask will be included. However, that is purely speculation. It could be someone completely unexpected. The writers have said that they want to utilize a more unknown villain."