Harvey Bullock is an old school tough guy detective. He's a donut-eating, cigar-smoking, loud-mouthed gumshoe who doesn't mind roughing up a suspect for a lead. When he and his partner play "good cop, bad cop," he's always the bad cop. And he's plagued by rumors that he's on the take or excessively violent with suspects. But for the most part, he does seem to be a good cop.
In the Officer Down storyline (spoilers to follow), Jim Gordon is shot, and after a series of twists and turns, GCPD finds out who did it, but the shooter enters the witness protection program to testify against the mob. Extremely loyal to Gordon, Bullock refuses to let an attempted cop killer walk free, so he gives up the shooter's identity to the mafia. When details about his involvement emerge, Bullock turns in his badge. He was seen from time to time as a private detective, but recently returned to the force (the details of how that happened are yet to be revealed).
Much like I confused Anna Ramirez in The Dark Knight with Renee Montoya, I thought Harvey Bullock was in The Dark Knight until the character's name was revealed as Det. Stephens. With Montoya, I wonder if there weren't issues with money (since Renee Montoya was created for the animated series, someone from that show might've been entitled to money if they used her in the movie), but Harvey Bullock was created in the comics, so Warner Bros. almost certainly owns his rights (WB, the studio that made The Dark Knight, owns DC comics).
Pros: He's a great character, well known, and like I've been saying, I'd like to see a good cop from the comics included in the movies.
Cons: Although I think his character was somewhat of a novelty in superhero comics, I think the cop who plays by his own set of rules is well-worn territory in movies.
How I'd do it: Just bring him on as a cop in the hunt for Batman.
Casting Guesses: There's actually a thread on the superherohype.com boards with casting ideas. Forest Whitaker is an interesting thought, though Bob Hoskins might be the best in terms of fitting the character (see Who Framed Roger Rabbit? for a perfect Harvey Bullock). But James Gandolfini was the first name I thought of, and he would probably offer the right mix of being faithful to the comics while adding something new.
Verdict: I'd love to see him, and can't think of a compelling reason to exclude him.
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.