Dr. Jonathan Crane, psychologist and fear expert, dons a Scarecrow costume and messes up Batman with some crazy fear-inducing drugs. Hey, wasn't this guy already in Batman Begins, you ask? And The Dark Knight too, you also ask? Well, fictional reader, you're just full of questions, but we'll get to that.
Pros: We already know he works in these movies. Solid character, fits with the level of realism they've established, nifty special effects to represent the toxin in action.
Cons: We've already seen him, so it's a less exciting choice.
How I'd do it: Based on his brief appearance in The Dark Knight, Crane is still pushing his wares all over Gotham. Add fear drugs to already crazy escaped mental patients, and that might be a recipe for the wackier villains the Batman comics have to offer. So if you want to have a bird fetishist with an arsenal of umbrella weapons running around Gotham City, the Scarecrow is a great way to set the stage.
Casting Guesses: Cillian Murphy, obviously. Doesn't the main picture on his imdb page look a lot like Tom Welling from Smallville? That has nothing to do with anything, I just always thought it was weird.
Verdict: A definite possibility. He was a means to an end in Batman Begins and little more than a cameo in The Dark Knight, he could play a bigger role in the third movie, or have another cameo to set up the even crazier villains.
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.