Pamela Isley is a botanist who at some point becomes poisonous and is skilled at manipulating men through sexual attraction. That is about all that's consistent about Poison Ivy's characterization over the years in the comic books. In some versions, she has the ability to control plants. In some, she emits toxic fungus spores wherever she goes. Sometimes she's an environmental terrorist, sometimes she just wants to be left alone with plants. She's shown contempt for all people in some versions, but other times she's partnered with Harley Quinn. Sometimes she's able to mess with men's heads because she's developed pheromones, sometimes it's a byproduct of her poisonousness. So there's a variety of ways to go with this character for a Dark Knight sequel while still being faithful to the comic books.
Pros: To be completely superficial, Poison Ivy has to be a smoking hot redhead, and that's never bad.
Cons: Her appearance in Batman & Robin is a huge negative. Uma Thurman wasn't all that bad, but it was a poorly written part and an all-around terrible film, and the public still associates the Poison Ivy character with that movie. And like many villains, her more supernatural powers don't fit in the realistic universe of the film series.
How I'd do it: Bruce Wayne becomes infatuated with Pamela Isley, improbably attractive scientist, while Batman battles an eco-terrorist using the alias Poison Ivy. No poisonousness, no pheromones, no plant powers.
Casting Guesses:Christina Hendricks is perfect for the part. She was wonderfully wicked as Saffron on a couple episodes of Firefly, and I can't take my eyes off her on Mad Men. In fact, if Poison Ivy is chosen and someone other than Christina Hendricks plays the part, I'd be pretty upset.
Verdict: Not impossible, but not very likely. The plotline I imagined turns her into a redheaded Catwoman who likes plants instead of cats. In that case, I'd just rather see Catwoman. And so would most people, I think.
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.