Jervis Tetch is really into Alice in Wonderland. Really into it. His backstory is a little muddled to me, but as of late he's portrayed as a guy who has somehow come up with mind control technology. In the animated series, he was a neuroscientist who invented it himself, in the comics I'm not clear on what his background is or how he acquired the tech (originally, in the 40s, he didn't even do the mind control stuff, and he was just a hat-themed villain). The catch with the mind control seems to be that the one doing the controlling gets some sort of mental feedback that drives him insane. So his fondness for Alice in Wonderland blossoms into a whole career as the Mad Hatter, building mind control hats for nefarious schemes. For most of his existence, he's been a jolly old crazy guy who just happens to be a criminal, but semi-recently (beginning here, I think), he's often been portrayed as a pedophile with a penchant for little blonde girls named Alice.
Pros: The pedophile angle has made him downright creepy, which elevates him above most other gimmicky villains.
Cons: He's still gimmicky. Mind control technology is a tough fit in the environment they've built the films in. Molesting kids might not fly in a PG-13 movie either.
How I'd do it: Ditch the mind control. Tetch is an escaped Arkham inmate who gets hold of some drugs tainted by the Scarecrow, and he goes a new level of nuts. He kidnaps or murders a couple little girls named Alice. The pedophilia is implied, but never stated outright (which might make it that much creepier). Maybe there's a police sketch of him in a hat, and the press label him the Mad Hatter. He can quote Alice in Wonderland and wear the hat just so fans will recognize him, but I really can't see the more overtly gimmicky elements of the character working.
Casting Guesses: Dustin Hoffman already looks like Jervis Tetch, so he's the obvious choice, and although he's not the actor he once was, I still think he can put together a really disturbing character. If not, someone unsettling, like Clint Howard, but not actually Clint Howard. Garret Dillahunt maybe?
Verdict: Possible, but doubtful. The version I threw together barely resembles the Mad Hatter of the comics, so announcing him as a central villain, then changing him so much might be disappointing to fans. If he appears, it might be hard to make it more than a cameo.
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.