One of Batman's oldest nemeses, The Monk is a good old fashioned vampire who wears a red cloak and red hood with a gold skull and crossbones on it. Among his powers: turning others into vampires, mind control, and just generally being hard to kill. He has a henchwoman of sorts in Dala, who isn't a vampire, but hopes that by serving the Monk, he will turn her into one.
Although he first appeared in 1939, the Monk hasn't been seen all that often. Most recently, he was featured in Matt Wagner's Batman and the Mad Monk.
Pros: He's a relative unknown, but has a very striking costume and loads of story possibilities.
Cons: He won't generate the same type of buzz as Catwoman or one of the other villains people remember from the earlier movies or the Adam West series. He's entirely supernatural, which is an element the movies have been avoiding.
Also, I'm not sure how the hood thing works in a live action movie. I have issues with certain masks and sometimes even heavy makeup in live action movies, where it removes the facial expressions that actors rely on to sell their performances. I hated the Green Goblin mask from Spider-man, every time Willem Dafoe put it on and spoke, I felt a total disconnect, like it was some guy in a suit while Willem Dafoe stood off camera saying his lines. And any time I'm thinking of the characters as actors on a set, I'm not all that engaged in the movie.
While these types of hoods are common in comics and cartoons (Yu-Ti, X the Eliminator), I don't think I've ever seen one in a live action movie. I suppose the G.I. Joe movie will be the test case for me there.
How I'd do it: No avoiding the supernatural here, so I'd just go right for it. GCPD finds a body and there's some circumstantial evidence pointing to a spooky house on the outskirts of Gotham. They can't get a warrant, but this one strikes Gordon as Batman's kind of case. Even though he's public enemy #1, they've worked out a way to discreetly communicate, and Batman heads to the house. It looks like something out of Dracula, but Batman confidently laughs it off, knowing that all he'll find inside is just a man. A crazy man, but just a man. Instead he faces a hooded vampire in a battle that he nearly loses. This shakes him up pretty badly, and forces him to realize that he's only prepared to fight men, not monsters, and heads off to study the supernatural (with Zatara? Zatanna? Probably can't use John Constantine, though), clearing the way for all kinds of craziness if they made a 4th movie.
Casting Guesses: With the hood, the look of the guy doesn't matter much. That means you have to have someone with a cool British accent. I'll say John Hurt.
Verdict: Normally I'd say no chance to anyone this inherently supernatural. But the Monk was one of the original supernatural Batman villains, so if you wanted to open the door to vampires and werewolves and magic, who could be better?
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.