The Reaper was the main villain in Batman: Year Two by Mike W. Barr and several artists. Gotham City's original costumed vigilante, predating Batman by many years, Judson Caspian took up crime fighting when his wife was murdered in a robbery. Unlike Batman, he took a more violent approach, often using lethal force.
In Year Two, one of my least favorite Batman stories ever (though some people do think highly of it), Bruce Wayne falls in love with Caspian's daughter while Batman tries to stop the Reaper, who's returned to the streets after a long absence. Inexplicably, Batman teams up with Joe Chill, the guy who killed Thomas and Martha Wayne, to stop the Reaper, and Batman even carries a gun for a decent portion of the series. So... not my favorite.
But as much as I didn't like the story, the Reaper wasn't a terrible character. And the folks behind the Batman animated series thought so as well, using parts of the Reaper character for their Mask of the Phantasm direct to video movie.
Pros: The introduction of the Reaper's daughter is an easy to way to bring in a love interest. The violent, Punisher-style vigilante forces Batman to examine the rationale of his non-lethal methods. Isn't there a point where you realize someone just won't be rehabilitated and you're just better off killing them? Or is the chance that you're wrong worth risking all those lives the villain will probably take in the future? Decent questions to center a movie around.
Cons: The scythes and the skull helmet and everything are kind of cool in a comic book way, but I think they'd look pretty silly on screen.
How I'd do it: Skip the Joe Chill element, and just have an old school vigilante come back to town, and have his daughter come as well.
Casting Guesses: An older guy, but still tough. Sam Shepard?
Verdict: It could happen. I'd prefer it didn't, because it would make me want to read Year Two again, which I would rather not do.
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.