Wasn't there talk of Ra's Al Gul's daughter coming for revenge on Batman because of her father's death when they were discussing The Dark Knight?
Personally I'd still like to see Catwoman - after all, if the Joker and Two-Face characters can be moulded to fit Nolan's take on Batman, there's no reason why Catwoman can't.
The 'organised crime' element has been pretty well established though in both films, first via Tom Wilkinson's character (with Scarecrow and Ra's Al Gul), and then Eric Roberts's character (with elements tied into Harvey Dent's storyline and Joker to an extent), so I imagine whatever villain they go for next will somehow tie into that.