straight from wikipedia (i know, i know)
Nolan is not committed to another sequel, explaining that he does not normally line up projects after completing a film. Gary Oldman is confident Nolan will return,[25] and Bale said he would return if Nolan did.[26] Oldman hinted in the third film Gordon would have to hunt down Batman.[27] He has also mentioned that the villain of the upcoming film may be The Riddler. [28] Nolan explained that as long as he is directing, he is not including Robin in the franchise because Bale is portraying a "young Batman", which meant "Robin's not for a few films". Nolan jokingly listed Frankie Muniz as a potential choice, if the studio forced him to cast.[29] Bale joked "I'll be chaining myself up somewhere and refusing to go to work" if Robin is introduced.[30]
In addition, Nolan considered the Penguin difficult to portray on film, explaining, "There are certain characters that are easier to mesh with the more real take on Batman we're doing. The Penguin would be tricky."[31] Christina Ricci, Sarah Michelle Gellar and Kate Beckinsale have shown interest in playing Catwoman,[32][33] while David Tennant wishes to play the Riddler.[34] Prior to the release of The Dark Knight in 2008, David Goyer ruled out using the Penguin or Catwoman as a villain in a future film, prefering to use antagonists from the comic that had not yet been portrayed on the big screen.[35] After Heath Ledger's death in January 2008, it is unknown whether the Joker will come back, be recast, or retired out of respect for Ledger.
http://en.wikipedia.org/wiki/Batman_%28film_series%29