Dr. Kirk Langstrom is losing his hearing. As a geneticist, he thinks he can give bats' heightened sense of hearing to people, curing not only his own ailment, but deafness entirely. He thinks he's close to completion, but his obsession with the project begins to cause problems with his marriage and eventually the funding for his research is cut off. Like most every other tragic/mad scientist in fiction, he decides the only way to prove himself right is to dangerously test his experiment on himself. It works perfectly at first, but obviously from the image above, he ends up turning into a half-man, half-bat monster. The beast still retains some of Langstrom, particularly his affection for his wife, but he mostly likes flies around Gotham, scares the crap out of people, and occasionally gets blamed for (but is not usually guilty of) murders.
Pros: Villains that reflect aspects of Batman work very well, and Langstrom's transformation into a bat-monster does a good job of that. I also have a personal connection to the character, since the first comic book I ever read featured Man-Bat.
Cons: Like Mr. Freeze, the science fiction aspects that won't work in a realistic movie are integral to the character.
How I'd do it: There have been some good Man-Bat stories over the years, but I don't think you can do the Man-Bat transformation, so I don't think you can do any version of the Man-Bat.
Casting Guesses: My initial instinct was Steve Buscemi, since the actor would have to be a little bit nerdy. But he also has to be sympathetic, and Buscemi usually excels more at being a little creepy. Maybe Alan Tudyk? He's slightly nerdy, sympathetic, and a great actor.
Verdict: I wish it could happen, and again if the movies had chosen a different style it really could have, but the Man-Bat just doesn't fit.
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.