Black Mask is one of the many Batman villains who has a sort of anti-Batman origin. Roman Sionis' parents were wealthy jerks who eventually drove their son to kill them both. He took over the family business and ran it into the ground, then turned to a life of crime only to discover that he was quite good at it. After running a crime syndicate with a mask on, he was disfigured in a fire, and his Red Skull ripoff appearance became permanent. From No Man's Land to War Games, two of the biggest Batman events of the past decade, Black Mask was a major player in the comics (probably most notable for brutally killing Spoiler (sort of)), but for the past couple years he's been more involved in Catwoman's solo series. See wikipedia for more.
Pros: A non-superpowered villain who's basically just a meaner version of a traditional mob boss, Black Mask could fit in the Batman film universe without much change to his origin. Though derivative of a more famous Captain America villain, the black skull is a cool visual.
Cons: His origin story isn't terribly interesting. And part of The Joker's story in The Dark Knight was offering his services as a crazy killer to the mob, which is along the lines of what Black Mask would do. So it might be too similar to do in back-to-back movies.
How I'd do it: Not much would really have to be altered. Flashbacks to Roman and Bruce as kids (hopefully Linus Roache wouldn't be too busy with his TV gigs to reprise his Thomas Wayne role), Roman's parents are cruel, he grows up to be a criminal, disfiguring accident brings on the Black Mask thing, he does something horribly sadistic, then Batman kicks his ass. The End.
Casting Guesses: Being able to do crazy eyes is a must, since the Black Mask look doesn't give an actor much to work with but his eyes. James Callis is wonderfully crazy on Battlestar Galactica, but I've never actually heard him do an American accent. Jason Dohring might be too young to convincingly play a childhood friend of Christian Bale's, but he'd be a good fit for the part. I'm out of ideas though. Help me out in the comments.
Verdict: Possible, but not very likely. Black Mask, unlike Calendar Man, fits in the Batman Begins/The Dark Knight environment, but he just isn't that interesting.
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.