First appearing in Death and the Maidens, Nyssa Raatko is the daughter of Ra's al-Ghul and half sister of Talia. Born in 18th century Russia, Nyssa did not know her father as a child, but her mother told her stories of Ra's. Eventually, Nyssa set out to track her father down. When she eventually managed to find him, Ra's was so impressed with her that he gave her access to a Lazarus Pit to extend her life, hoping she would have a son who could be his heir. Learning alchemy from Ra's, she was able to reuse the pit.
However, during World War Two, Nyssa ended up in a concentration camp, where medical experiments left her infertile. She begged for Ra's to free her, but he abandoned her. Obviously angry, Nyssa hatches a plan to drive Talia insane through repeated deaths and resurrections by the Lazarus Pit, then assassinate both Superman and Ra's al-Ghul.
Pros: Ties back into the first movie. Gives a feminine presence without introducing a love interest, which might be an interesting way to go about things.
Cons: Her story relies heavily on Lazarus Pits, which the movie version of Ra's al-Ghul skipped entirely. Talia is the more popular al-Ghul daughter.
How I'd do it: If I was including Nyssa, I'd have Talia as well. Have Talia leading the League of Shadows, coming back to Gotham for revenge, while Nyssa is determined to take the League of Shadows (and her sister) down.
Casting Guesses: Depends on who you pick for Talia, since they'll have to look somewhat alike. Going by the comic, she sorta looks like Megan Fox (kinda nsfw-ish) a little. Or maybe not that much, but it gave me an excuse to hunt down those pictures.
Verdict: I guess there's an outside chance for a cameo type appearance if Talia's the main villain, but I wouldn't count on seeing her at all.
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.