Crispus Allen was a cop from Metropolis who was hired by the Gotham City police department to be Renee Montoya's partner after Mackenzie Bock was promoted. Allen, coming from a city with a superhero who tended to work within the law, didn't like Batman's brand of vigilante justice. But after spending some time in Gotham, he learned just how crazy the city was, and that by the book police work wasn't going to keep the streets safe. But he never saw Batman as anything better than the lesser of two evils.
While working a case, Allen suspected a crime scene investigator of tampering with evidence and began investigating him. The man was tipped off that he was being investigated and murdered Allen, then used his experience with crime scenes to ensure that he wouldn't be caught. Allen was brought back to life by the Spectre, a spirit that resides in a human host, trying to bring justice to the world. I'm not big on those kinds of stories, so I stopped paying attention to Allen at this point, so check out wikipedia for what happened after that.
Pros: A good Gotham cop with a unique view of Batman would be a welcome addition.
Cons: He's a very minor character. His origin, though easily changed, ties to Metropolis, and from reading that interview with Christopher Nolan a while back, I think his vision of Gotham doesn't exist in the same world as Superman.
How I'd do it: Like every other cop, just have him join up with Gordon. Possibly out of Gordon's attempts to find the only good cop(s) in Gotham. But I would skip the Metropolis bit.
Casting Guesses: Don Cheadle's the obvious answer, since he really looks the part (and he's a fantastic actor), but I doubt Allen would be a big enough role for Don Cheadle to consider. Gary Dourdan from CSI provided Allen's voice in the Gotham Knight animated feature that came out around the same time as The Dark Knight, and he wouldn't be too bad either.
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.