Haven't done a villain in a while, so it seemed like a good time for Prometheus. Yet another of those anti-Batmen, Prometheus' criminal parents were killed by a cop when he was young. He raided their supplies of cash after their death to fund his travels around the world, training to be the best criminal he could be. He invented a device that let him download fighting styles into his brain, allowing him to defeat almost anyone hand to hand.
Prometheus carefully planned an assault on the Justice League headquarters, defeating the Flash, Martian Manhunter, Batman, and others, but ended up being defeated because he was unprepared for Catwoman, who had broken into the Justice League hoping to steal some stuff. So despite his almost Batman-like penchant for preparation, an unexpected event can really throw the guy off.
Pros: Contrasting Batman with the villain he faces is kind of nice.
Cons: Prometheus' origin is too perfectly opposite Batman, it stretches disbelief that a person would mirror Bruce's life so completely. Also, his equipment is far too science fiction-y for what we've seen in Batman movies so far.
How I'd do it: A master criminal who's as obsessive and skilled as Batman. He doesn't need to be Batman's perfect polar opposite.
Casting Guesses: I can't even remember what the guy looks like with no mask on, so I guess it doesn't matter too much as far as looks go. Maybe Jeffrey Donovan, the guy from Burn Notice?
Verdict: Nah. The character's too new, too contrived to be Batman's opposite, and too attached to his advanced technology to work in a Batman 3.
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.