Julian Day (get it?) is crazy, obsessed with dates, and commits crimes with calendar/holiday/date themes.
Pros: Christopher Nolan and friends have cited Jeph Loeb and Tim Sale's The Long Halloween (pictured above) as a major influence, and the Calendar Man is one of the featured villains in that series.
Cons: Uh... read the description again, that's one giant con. Not long ago, IGN rated him as the worst Batman villain of all time. Venture Bros. fans may remember Mister Monday from early in season two, who helped show just how ridiculous the calendar theme is ("Oh, this is going to be so Mondaylicious!").
How I'd do it: I wouldn't.
Casting Guesses: I'd hate to inflict an awful role on anyone, but I think if you did someone like Calendar Man, you'd have to play it over the top, with lengthy, calendar-infused proclamations of Julian Day's criminal genius in a classic comic book villain style. And anyone who's ever seen A Knight's Tale knows that Paul Bettany can ham up a monologue better than anyone.
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.