Recently Warner Bros. set the official date for the third Batman movie to be July 20, 2012. It’s the only concrete detail on the movie while it’s under the presumption that Christian Bale and Christopher Nolan will be involved in the film.
The Riddler is a fan favorite for the next villain, though I’m hoping for the Riddler to be an intellectual rival to Batman as a detective. The third movie can easily suffer the fate of most trilogies if it follows the same formula of “bigger is better” — having the Riddler as a rival on top of an actual villain would help keep the film grounded, and further display the detective aspect of Batman that was only touched upon in the prior movies.
As for actual villains: I’d like to see Bane and Black Mask being used. If Bane’s characterization is the intelligent version like in the comics — coupled with Black Mask who hates Bruce Wayne and eventually controls much of the underworld of Gotham — they would make a formidable team against Batman while he’s in his rivalry and occasional help from the Riddler.