Jean-Paul Valley's story is pretty complicated, but here's the quick version. His father was a super assassin type guy for the Order of St. Dumas. His father secretly trained Jean-Paul from birth with subconscious commands, called "The System," on how to be a super assassin too. When his father dies, Jean-Paul becomes Azrael. On one of his first tasks, Azrael runs across Batman, and decides to ditch the assassin gig and join Batman with the superheroics. Batman grows to trust Azrael so much that when Bane breaks his back, Batman asks Jean-Paul to take over. The only problem is that The System is constantly fighting for control of Jean-Paul's mind, and he becomes more and more violent, violating all of Batman's principles. Eventually, Bruce recovers and has to take back what they constantly referred to as "the mantle of the Bat" by force. Not long after, Azrael was presumed dead in the comics and Jean-Paul Valley hasn't been seen since, but I keep hearing rumors of his imminent return.
Pros: Personally, I hated the character and every story he was involved in. Drop a comment if you liked him and why, please, cause I'm stumped.
Cons: He was very 90s-comics. Big muscles, mental anguish, fonts meant to imply spooky/scary voice, giant fireballs, stuff like that. He didn't have pouches strapped everywhere though, maybe that could go in the "Pros" section.
How I'd do it: You have to ditch the costume (or modify it so radically that it's unrecognizable) and ditch The System. There's a super secret organization called the Order of St. Dumas that doesn't like the whole Batman thing, so they send their best assassin, codenamed Azrael. But that's awfully close to the League of Shadows being pissed at Batman for killing Ra's al Ghul and having Talia send their best assassin, Lady Shiva. And the League of Shadows/Talia/Shiva story is much better, I think.
Casting Guesses: Jean-Paul's supposed to be European, I think (Swiss?), and in good shape. Karl Urban isn't European, but he's good at faking it. I think I suggested him for some other part, too, but oh well. Thomas Kretschmann is German but looks the part a little.
Verdict: I can't imagine any reason it would happen. I just didn't find him to be a character who was in any way interesting. Any Azrael-related story could be better handled by another existing Batman character.
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.