Raised by David Cain to be a killing machine, she was never even taught to speak, let alone read or write. But since instead of actual language, she had learned to fight, her native language is body language. This gives her the ability to anticipate her opponents movements. After years of training, her father sent her on a mission to kill a man when she was just a girl. The event scarred Cassandra, and she ran away from her father, determined not to kill again. After years of guilt, she was eventually discovered by Barbara Gordon, who gave her blessing for Cassandra to become the new Batgirl, and it didn't take long for her fighting skills to impress Batman as well. She flourished early on as Batgirl, even learning to speak, but her story ended up taking a series of bizarre turns. You can read two pages that attempt to explain the whole thing here, but basically she got drugged by Deathstroke, turned evil, killed a bunch of people, injected with a cure for the drug that turned her evil, took off before it could fully un-evil her, and did some more evil stuff before finally becoming good again. It was pretty dumb.
Pros: The next Batman movie almost certainly needs a female character, and Cassandra Cain is a lady.
Cons: Barbara Gordon is Batgirl to anyone but hardcore fans. If Batman were to take on a sidekick, it should probably be Robin. You probably need some David Cain and Lady Shiva backstory to introduce Cassandra, and all three of those characters' introductions would eat up most of the movie, not leaving a whole lot of Batman time.
How I'd do it: Flash back to Batman training with Lady Shiva, at one point drop a scene where he sees a picture of her pregnant or something like that, and he asks her about it but she dodges the question. Flash forward to the present, Lady Shiva's been hired to take out Batman, and out of nowhere he's saved by Cassandra. A quick explanation that she'd been searching for her mother, only to discover that she was a killer, and I think set up David Cain to be a future villain, instead of spending any real time on him then.
Casting Guesses: Zhang Ziyi I guess.
Verdict: I almost fell asleep trying to come up with that plot idea, it would be just that boring. Add that to all the other negatives, and there's no way we'd see her.
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.