Victor Zsasz is just a regular old serial killer. No supernatural powers, no elaborate costume, no obsession with birds or plants or cats, no lifelong hatred of Bruce Wayne. He just likes to kill people. The only thing that separates him from Dahmer or Gacy or any other serial killer is that Zsasz carves a mark into himself for each victim. Most drawings make him look like he's well into triple figures, which I have to think would be some kind of record.
Pros: Zsasz is different from your standard Batman villain. He's frightening in a much more realistic way.
Cons: He's not too much of a physical challenge for Batman, to do his murders justice might make it hard to keep the PG-13 rating of the past two movies, and he doesn't really make for exciting chase scenes or dramatic rescues.
How I'd do it: Technically, we've already seen Zsasz. He appeared briefly early on in Batman Begins, portrayed as a mob hitman, with Rachel Dawes trying to keep him in regular prison and out of Dr. Crane's care. He later shows up among the Arkham escapees and his trademark scars can be seen if you look closely. The transition into full blown serial killer is an easy one, since we can assume he was subject to the Scarecrow's experiments, and now he's roaming the streets of Gotham. Zsasz is another guy who can't be the featured villain, so if he shows up I'd keep his appearance brief.
Casting Guesses:Tim Booth was just fine as Zsasz in the first one.
Verdict: He could get a token appearance, but again there's no way he's the main villain in Batman 3.
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.