A body is dumped into a Gotham City swamp, then fifty years later the body arises, disfigured, super-strong, and with no memory of his former life. Basically a big hulking zombie, Solomon Grundy has varied in strength and intelligence over the years. In The Long Halloween (pictured), where he strikes up an unusual friendship with Harvey Dent, Grundy is barely able to say more than "Solomon Grundy, born on a Monday," and though he's very strong, Batman is able to defeat him in combat. In other versions, he's not exactly smart but he can carry on conversations, and he's sometimes so strong that Superman has trouble with him.
Pros: Batman hasn't fought anyone freakishly strong in the movies yet. And many people have fond memories of Grundy, either from the comics or from being part of the Legion of Doom on The Superfriends cartoons. And visually, he's pretty cool looking.
Cons: Zombies are a stretch, realism-wise. The movies have favored the psychological over the physical, and Grundy's lack of intelligence doesn't fit well with that. Finally, Solomon Grundy has faced off against many other DC superheroes, so Warner Bros. bosses might want to save Grundy for a Green Lantern movie, where his supernatural origins might fit perfectly.
How I'd do it: I guess the best way to do it is to have a very big and very strong guy beaten to a pulp and left for dead. He survives, but with massive brain damage he can't even remember his own name. One of his origin stories has him only remembering that he was born on a Monday, and someone suggests Solomon Grundy as a name, referencing a nursery rhyme that I'd never heard of before reading it in the comics. So you could go with that. You're left with a big strong guy who's angry at the world because he can't remember it, which might be enough to fill a TV episode, but I don't think you can stretch two hours of plot from that.
Casting Guesses: The usual actors I suggest for big guy roles: Michael Clarke Duncan, The Rock, etc.
Verdict: Nah. Like about half of Batman's villains, they're quality characters, but the movie franchise's choice of realism makes it hard to do supernatural bad guys.
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.