Batman needs a love interest to keep him anchored. As I stated a few pages ago the purpose of Rachel Dawes was to humanize and anchor Bruce Wayne, to give him a reason to think that maybe there was a chance that his mission could end and he could actually get to be happy like everyone else. "The Dark Knight" proved though that while this notion was well meaning simply isn't possible for someone as driven and committed to the course of action that Bruce is. The casting of female characters continues to provide that sense of humanism for Bruce and to create conflict and drama. Female characters are almost a necessity for a character like Batman in my opinion. Especially if you're not planning on introducing Robin yet, jokes aside, Dick Grayson is another concept of the humanizing plot device.
I wouldn't be surprised to see in the opening bits of the film that not only is Bruce still mourning the loss of Rachel, but without that person keeping him grounded he's become violent and reckless. I suspect that the characters that Nolan plans on using will refocus Batman and in the end ultimately provide Bruce Wayne with a sense of balance, a literal Rising of his acceptance...both for him and for Gotham Cit.
And Selena will likely represent the thrill of being Batman/Bruce Wayne.