Mr. Freeze, until the 1990s, was a silly gimmick villain who carried a freeze gun and committed ice-related crimes. If you ever saw Otto Preminger play Mr. Freeze on the Adam West Batman TV show, that's pretty much it. Then the animated Batman series reinvented the character completely. Victor Fries is a scientist specializing in cryogenics whose wife is terminally ill. He puts her in cryo-statis until they can find a cure, but badly damages himself in an experiment. As a result of the accident, his body requires sub-zero temperatures to survive. He builds a suit that keeps him cold (which runs on diamonds, a call back to the "ice" heists he used to pull in his gimmicky days), makes him strong, and has a gun that allows him to shoot freeze rays.
Pros: Still trying to save his wife and willing to do anything for her, he's a tragic villain who makes for some great stories.
Cons: The freeze gun, the cryo-suit, and the need to stay at sub-zero temperatures don't fit the movies at all. Plus, the public still associates the character with the awful Arnold Schwarzenegger portrayal in Batman & Robin.
How I'd do it: It's very, very tough. He can't do most of the freeze-related things. A man willing to rob and murder to save his wife is compelling, but is that Mr. Freeze anymore?
Casting Guesses: Patrick Stewart would be great. Great.
Verdict: In a more fantastical Batman film universe, a well-done Mr. Freeze would make for a wonderful movie, but in the universe Christopher Nolan and friends have established, I just don't think Mr. Freeze fits.
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.