Arthur Brown is obsessed with clues, and he intentionally leaves behind clues at all his crime scenes. Though his clues are technically not riddles, he is basically a poor man's Riddler.
His main contributions to Batman comics are forming the Injustice League with a group of lesser-known villains, and fathering Stephanie Brown, aka Spoiler, who briefly served as Robin.
Pros:You can take most potential Riddler stories and turn them into Cluemaster stories.
Cons: But they wouldn't be as good.
How I'd do it: I probably wouldn't. Any Cluemaster story would feel like a cheap version of a Riddler story. And having him as a cameo would require too much time to explain his impulse to leave clues behind.
Casting Guesses: I tend to think of him as an older guy, father of a teenage daughter. Character actor Gregg Henry could fit the part.
Verdict: Not going to happen. If they ever started making Robin movies featuring Tim Drake, he'd probably have to show up. But in Robin-less Batman, he's a nobody.
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.