Hush was the main villain in Batman: Hush, a very popular comic book storyline from 2002, and since the mystery of his identity was a main element of the story, many Hush spoilers will follow...
The Elliots were close family friends of the Waynes, and Tommy Elliot was young Bruce Wayne's best friend. But while Bruce adored his parents, Tommy wanted to be free of parental control and to have access to his family fortune. So Tommy attempted to murder his parents by cutting the brake line on their car. Though his father was killed, Dr. Thomas Wayne was able to save Tommy's mother. Not only would he not inherit their money, but Tommy was now forced to help care for his mother, a situation he blamed on Thomas Wayne. Later, when Thomas and Martha Wayne were murdered, Bruce had everything Tommy had ever wanted, so Tommy redirected his anger at Bruce.
Despite his many childhood issues, he somehow managed to become a relatively productive person, becoming an accomplished surgeon. He came to treat the Riddler, who had just deduced Batman's secret identity. Learning that they shared a common enemy, Elliot wrapped his head in bandages, started calling himself Hush, and he and Riddler hatched an elaborate plan to get at Batman. Obviously it didn't work, but Hush has returned a few times since, still armed with the knowledge of Bruce's double life, and is able to cause a lot of trouble in Gotham.
Pros: His origins are tied with Bruce Wayne's, so digging through Tommy Elliot's origins would allow for some good scenes with young Bruce. He has no supernatural powers, so he fits with the movies' realism.
Cons: It's a bit of a stretch to buy an angry surgeon as a real threat to Batman, even if he does know Batman's identity. Hush in the comics relies on manipulating a number of Batman's costumed enemies so that they can actually hurt him, but the movie Batman only has a few enemies still alive to do anything with.
How I'd do it: Tommy Elliot would have to hook up with Coleman Reese. Learning Batman's identity is hugely important for turning Elliot into Hush, and Reese is the only one who might possibly let it slip. Then I guess you'd have him attack Batman through Bruce's friends, who at this point seem to only be Jim Gordon, Alfred, and Lucius Fox.
Casting Guesses: Under the bandages, Tommy Elliot's a redhead, so Damian Lewis (the TV series Life) is the first name that came to mind. Not just because he'd be good for the role (which he would be), but because I'm blanking on other red haired actors of appropriate age.
Verdict: It could happen, and though technically he should probably be considered a good fit, I just don't like him that much as a villain. I got most of the way through the Hush storyline without realizing Hush was a person. And then they started running a campaign to have people guess who Hush's identity was, and I hadn't realized I was supposed to be guessing. That left me somewhat unhappy with the character. I'm warming to him a little through the current storyline in Detective Comics, so maybe as that progresses I might change my mind, but for now I'll just say he's a possibility.
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.