Whisper A'Daire is at least eighty years old, but looks pretty good for her age. Her boss, Ra's al-Ghul, gave her a serum by her boss to make her effectively immortal, which also gave her some kind of shape shifting snake powers. She sheds her skin to keep looking young, can do some kind of mind control stuff, and at least at one point could spit acid or venom or something. She's also given Ra's serum to her henchmen, like Kyle Abbot (pictured), who changes into a wolfman.
Pros: With the demise of Rachel Dawes, Batman probably needs a lady friend for the next movie. And she ties back in to Ra's al-Ghul from Batman Begins.
Cons: Ra's in Batman Begins didn't have the Lazarus pit or anything, so him giving some lady an immortality serum doesn't make much sense. Also, nothing supernatural has happened so far, so a snake lady with mind control doesn't really fit.
How I'd do it: I guess have her as a follower of Ra's, after revenge for Batman foiling the League of Shadows' plan to take down Gotham. Plus she can be a love interest or a femme fatale.
Casting Guesses: Not too many redheads out there. I already suggested Christina Hendricks for Poison Ivy, and she's not much of a fit, physically at least, for Whisper A'Daire. So I'll go with Alicia Witt.
Verdict: I say no chance. Not that she's a bad character, but Talia al-Ghul can bring the same call-back to Batman Begins and serve as a potential love interest, but she isn't based around supernatural abilities and is much better known.
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.