Denver and Dumfree Tweed are cousins who look not only like identical twins, but also like Lewis Carrol's characters Tweedledee and Tweedledum. Not generally affiliated with Mad Hatter even though they're all based on Alice in Wonderland characters, Tweedledee and Tweedledum are strategists who let henchmen carry out their plans. Recently they did hook up with the Mad Hatter and formed the Wonderland Gang (no relation to the actual gang), adding some new Alice in Wonderland-themed characters.
Pros: Nothing supernatural, they're just criminals looking for money.
Cons: The Mad Hatter's a much better (and better known) Wonderland villain. Tweedledee and Tweedledum haven't been involved in many of the bigger Batman stories, so they wouldn't generate much fan interest.
How I'd do it: Have the Tweeds be low level criminals who had been stopped by Batman in the past. When they see Batman on the run from the cops, they think it's open season for crime and set out on their own.
Casting Guesses: Two portly gentlemen who look alike... the only actors who are coming to mind are comedic ones. Jack Black types who would be more than a little out of place in a Christopher Nolan Batman movie. I'm sure there are a couple of perfect guys for the part, but I doubt they'll get the chance to prove it.
Verdict: Nope. When there are two similar villains and one is just that much better than the other (like Catman and Catwoman), I don't see any reason to include the crappy one.
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.