The identity of the Holiday killer was the central mystery of The Long Halloween, so there are lots of spoilers ahead. And it's a good read, so don't spoil it for yourself if you haven't read it already.
Holiday was the name of a serial killer who struck only on holidays. Holday's targets were mobsters, at first members of the Falcone family, but later targeting the rival Maronis (you may remember Tom Wilkinson as Carmine Falcone in Batman Begins, or Eric Roberts as Sal Maroni in The Dark Knight). Holiday struck with a .22 caliber pistol, the serial number filed off, and a baby bottle's nipple used as a silencer. At each crime scene, Holiday would leave behind a holiday-appropriate item as a calling card (a jack-o-lantern on Halloween, a snow globe on Christmas, etc).
The initial suspects were all mobsters. Calendar Man was obviously questioned, since it's his kind of crime, but he was locked up in Arkham the whole time. Eventually, Alberto Falcone, the somewhat neglected son of Carmine who had been Holiday's New Year's Eve victim, reappeared, very much alive, and confessed to the crimes. But later, as Two-Face is being arrested for a separate crime, he says to Batman and Gordon, "You both know, don't you? There were two Holiday killers." Batman assumes it's a side effect of Two-Face's obsession with duality, but we're then shown a scene of Gilda, Harvey Dent's wife, disposing of evidence and confessing to the first three murders to herself. Harvey had told her once the Falcones were taken down, he would have time to start a family, so she'd decided to hurry that process along. When someone else started committing Holiday murders, she assumed Harvey had realized what she was up to and taken over, but in fact it was Alberto Falcone.
Or at least that's how I read it. This guy claims that Alberto was the killer the whole time, but I don't buy it.
Pros:The Long Halloween was a big influence on both of the first two movies, and Holiday targeting gangsters fits with the movies making everything mob-related.
Cons: There's no Gilda Dent in the movies. And the Holiday thing fits better in an episodic format (monthly comic books, a tv series, etc) than a single movie.
How I'd do it: You could have Alberto Falcone kill some people over the course of a year. Have Batman be the initial suspect. Call in The Riddler, private investigator, to figure out who did it. A fun time would be had by all, but it would be missing something, I think.
Casting Guesses: Alberto Falcone is kind of nerdy -- the kid who goes to Harvard is the black sheep in a family of gangsters -- but he's also angry and bitter. Ryan Phillipe could pull it off. Zachary Quinto too, but after playing Sylar, people might just automatically assume he's the murderer.
Verdict: I doubt it, but it's not impossible. There's been some talk about an animated adaptation of The Long Halloween, and doing that would present a more fun version of Holiday (see this and scroll way down for a mention of the animation possibilities).
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.