The Outsider was a villain from the 60s who had tormented Batman and Robin from afar for a while before they actually met. When they finally face off, Batman discovers that the Outsider has mental powers. He turns Robin into a coffin (don't ask), and Batman narrowly escapes the same fate before knocking the Outsider into a regeneration machine (click for a larger version):
Of course he looks familiar, Batman, that's none other than Alfred Pennyworth! Years earlier, I think because of paranoia over hidden messages in comics, they killed off Alfred (he pushed Batman and Robin out of the way of a boulder, only to be crushed himself) and brought in Aunt Harriet for a female presence at Wayne Manor. Then, with the success of the Alfred character in the Adam West TV show, they brought him back with a Princess Bride style solution: he was only mostly dead! Some scientist puts him in the regeneration machine, but it goes awry and the Outsider is born.
In classic comic fashion, after being turned back into his normal self by the pictured second exposure to regeneration rays, he remembers nothing of his villainous ways, and he goes back to Wayne Manor and everyone is happy again.
Pros: The Alfred character is hugely important to the Batman mythology, but sometimes it's hard to write him things to do besides be Batman's conscience when he returns home. And turning into the Outsider is a thing to do. It's classic Silver Age Batman.
Cons: Classic Silver Age stories are awesome in their own way and all (The Outsider's antics are collected in Showcase Presents volume 1 and volume 2), but have nothing to do with Batman Begins or The Dark Knight.
How I'd do it: Skip the part about Alfred dying. Skip the regeneration machine. Skip the mental powers and the ability to turn people into coffins. And don't call him the Outsider. I think I'm on to something here.
Casting Guesses: Michael Caine, naturally.
Verdict: No way. Unless my plan to leave Alfred completely unchanged counts.
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.