The Mutants were a gang from Frank Miller's The Dark Knight Returns. And if you haven't read that, quit reading my crap and go get it, and not just because this will be one big spoiler after another.
For some reason I'm striking out on decent images of the mutants (if I come up with one, I'll replace the above, which is of Batman on his way to kick the tar out of the leader of the Mutants), but physically they're notable for generally being young, their extremely pale skin, and their 80s quasi-futuristic sunglasses (the kind that sorta look like Geordi Laforge's visor from Star Trek, but with a single red lens stretched across both eyes). You can see them in this video from the Batman: The Animated Series episode "Legends of the Dark Knight." They speak with a unique brand of slang that's barely intelligible ("Batman don't shiv. He nasty. Balls nasty.").
In chapter 2 of The Dark Knight Returns, "The Dark Knight Triumphant," the Mutants, like most every other gang, want to kill Batman and Jim Gordon and take over the city. Which by itself doesn't seem all that interesting. What makes them unique is their reaction to defeat. Batman nearly loses his first fight with the Mutant's leader, saved only by Carrie Kelly (Robin). Though the battle results in the leader's arrest and imprisonment, the Mutants themselves continue to terrorize Gotham. Batman realizes that the only way to beat them is to humiliate them. He arranges for the leader to escape, and for all the Mutants to be at the same place at once to witness the rematch. Carefully choosing the location, and using what he learned in the first fight, Batman breaks the leader and shows the Mutants what will happen to them if they defy him. Their reaction? They become the Sons of Batman (and somehow I was able to find a picture of them, at left), painting the Bat-logo on their face and bringing vigilante justice to all of Gotham.
Pros: With Batman left as public enemy #1, winning over a gang and turning them from crime to crimefighting is a good way for him to be redeemed.
Cons: What passed for cool and futuristic in 1986 is neither cool nor futuristic now.
How I'd do it: It'd be hard to use the look of the mutants, especially the leader, the slang might not work, and they'd have to be made older (having violent kid vigilantes calling themselves the Sons of Batman is probably not the kind of thing parents' groups would react well to) but the plot can be used almost entirely. A gang of youths rises up wreaking havoc all over Gotham. Fugitive Batman takes down their leader in front of them all, and they're so impressed that they become his disciples.
Casting Guesses: There'd be dozens of them, so I'm not going to bother with casting guesses here.
Verdict: They're a good fit. My only concerns are the transition from Mutants to Sons of Batman, which was done rather abruptly in one panel in the comic, and how to let Batman command his army of former gang members in a way that's not reckless (recklessness is something The Dark Knight Returns wasn't especially concerned with).
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.