Waylon Jones was born with a condition that made his skin scaly, tough, and green. Ridiculed as a child, he grew up to be very big and very strong, and began wrestling crocodiles under the name Killer Croc. He headed to Gotham, deciding that crime was a better way to make a buck. He's been a thief, a mob enforcer, and he briefly ran his own criminal organization, which was odd since he's never been shown to be all that bright. Recently, his medical condition has been portrayed as progressive, slowly turning not just his skin, but his whole body and even his mind into that of a reptile. At this point in the comic books, he's usually shown as a reptilian monster living in the sewers of Gotham City.
Pros: Big scary dumb guys can make for some cool fight scenes.
Cons: The crocodile-man aspect of the character doesn't fit in the movie Gotham. And big scary dumb guys aren't as interesting from a storytelling point of view. With the Joker, Scarecrow, and Two-Face, three guys Batman can easily best in a toe-to-toe fight, the folks behind the movies have shown a preference for characters who are psychological threats more than physical ones.
How I'd do it: You can't make him actually turn into a crocodile, and probably can't even have him green, but I'm sure there's a real skin condition out there that could be used to make a movie Killer Croc frightening to look at. But he's too simple a character to really base a movie around, but like Firefly he'd make a decent villain for a side plot.
Casting Guesses:Michael Clarke Duncan seems to be the go-to actor for huge muscular guys. And he is huge and muscular (and a good actor, too).
Verdict: He could appear, but it would be next to impossible to base a whole movie around Killer Croc.
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.