Perhaps Bruce realizes he doesn't want either Talia or Selina but instead turns to john(edit) Blake. The films last act begins with a brokeback mountain scene and finally a marriage on Wayne Manors front lawn. Alfred burns the bat-suit, self destructs the cave and quits and drives far away,wanting nothing to do with the whole charade. Shaking his head in disbelief he accidently drives off a cliff while trying to take a swig of whiskey from his flask. Fade to black. The Dark Knight Rises: A christopher nolan joint. roll credits.
now THAT would be some fan reaction!
Really tho, I think Bruce will end up with Miranda Tate, who may or may not be Talia, get married and have a family....he'll stop being Batman and together they will continue the work of Thomas and Martha Wayne. Bruce has finally found peace, a new life and together they work toward a better future for Gotham City. Can't get more "bitter sweet" than that.