In the original comics history, Barbara eventually confesses her identity to her father and retires the cape, and successfully runs for Congress. After the continuity reboot of 1985, she was again Batgirl, but only for a short while before her career was ended in The Killing Joke.
Pros:A well known, well liked character. Provides the female presence that may be lacking going forward after the events in The Dark Knight.
Cons: Batgirl is in some ways like a second Robin, and the filmmakers have expressed no interest in Robin, so I find it hard to believe they'd find Batgirl any more interesting. And in a realistic setting, it's hard to justify Batman letting a young girl risk her life, especially Jim Gordon's daughter.
How I'd do it: Technically, Barbara probably appeared in The Dark Knight, in the scene where Two-Face holds Jim Gordon's family hostage (but she's credited only as "Gordon's daughter"). She's awfully young to start dodging bullets and jumping from rooftops, so I think I'd hold off for a movie or two.
Casting Guesses: A teenager part, which probably means a lengthy casting search. Unless there's a tough, smart redhead on one of those Gossip Girl type shows, and she can actually act.
Verdict: Not for Batman 3, but maybe sometime down the line. Not until after they break down and include Robin, though.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.