The Riddler, otherwise known as Edward Nigma, Edward Nygma, or Edward Nashton, is a brilliant man, obsessed with puzzles, mysteries, secrets, and, of course, riddles. Many people know him from the Frank Gorshin's portrayal on the 60s television series or Jim Carrey in Batman Forever where he wore a jumpsuit covered in question marks and seemed like a poor man's Joker, but more modern versions of the character have gone for a green suit and bowler hat, and a more complex personality. His crimes come from an obsessive need to prove his intellect. Leaving clues that he could solve allows him to test himself against Batman, hoping to prove to himself and to the world that he's superior. At one point, he deduced Batman's identity as Bruce Wayne and considered it the greatest riddle he'd ever solved, but concluded that the mystery was only powerful because no one knew, so he didn't tell anyone. In typical and frustrating comic book fashion, the problem of a villain knowing Batman's identity was resolved by giving the Riddler amnesia.
Recently, he's abandoned his criminal ways and become a private investigator. Though he's "reformed," he hasn't magically become a good guy. He's less interested in helping people than he is in making money, getting famous, and solving crimes before Batman can.
Before we continue, you should know that this is going to have a lot more Dark Knight spoilers than most of the articles I've written here. You've been warned.
Pros: With a history as both a criminal and a detective, the Riddler is very flexible and can be used in any number of ways. His modern costume requires little alteration to fit in the movie universe (a dark green suit and a purple tie with a question mark on it?). He's memorable, famous, and many fans, based on the crazy rumor mill going around, would love to see him on the big screen portrayed by anyone but Jim Carrey. And Gary Oldman (Commissioner Gordon) has suggested that we will see the Riddler.
Cons: Many people do only associate Jim Carrey and Frank Gorshin with the character, and their manic, over-the-top style would be a terrible fit with the realism established in Christopher Nolan's movies.
Casting Guesses: The internet is swamped with rumors that Johnny Depp will play the Riddler, but those rumors started with the National Enquirer, so I wouldn't take them too seriously. Plus at one point the internet rumor mills were saying that Anthony Michael Hall was playing the Riddler in The Dark Knight, which he obviously did not. This is not to say that Johnny Depp wouldn't do a good job, but my friends like to discuss who we'd cast as certain characters in movies, and our choice for most male characters age 25-50 is Johnny Depp, and he can't play every part. As much as I'd enjoy seeing him, I suspect that he'd be hard to attract to the part, or at least difficult to schedule.
There's also a question as to whether we've already seen the Riddler. In The Dark Knight, Coleman Reese, like the Riddler, solves the mystery of Batman's identity. And the Riddler is known for his goofy name pun, "E. Nigma = enigma," and many are quick to point out that "Mr. Reese = mysteries." So it's possible that the Riddler was already cast as Joshua Harto. Harto did an excellent job, but I suspect that we won't be hearing too much from the character in the future, whether he's this movie series' version of the Riddler or not.
As for my thoughts, I already suggested him for The Ratcatcher just because I couldn't think of anyone else, but Michael Emerson's character on Lost has some Riddler-like qualities, so I know he could do a great job. The ideal choice though is Guy Pearce, who previously worked with director Christopher Nolan on Memento. He looks the part perfectly, and has a demeanor that fits the Riddler just as well.
How I'd do it: Assuming, as I said above, that we haven't already seen the Riddler, I like the idea that Edward Nashton (Nigma/Nygma is too silly for these movies), a brilliant but shady private investigator, is unable to resist the mystery of unmasking Batman. Since the costumed vigilante is public enemy #1, Nashton could be hired by the city, or he could just come on his own for the publicity. Or an individual could hire him, either someone who wants to make his name by helping to bring down Batman, someone connected with organized crime who stands to make more money with Batman out of the way, or preferably both.
He could decide that the best way to lure the Bat out of hiding is to appeal to his vigilante nature and commit high profile crimes, leaving clues for Batman to follow. Or he could just play it straight like a normal detective. While Jim Gordon, knowing the truth, might try to intentionally slow the hunt for Batman or keep it on the back burner, Nashton picks up clues or otherwise fans the media flames, forcing Gotham City PD to act. And in the end, Batman could reveal the link between Nashton's employer and the mob, allowing him to at least partially redeem himself to the public.
Verdict: The Riddler's a great fit, possibly the best out of any villain. He won't be the version TV and movie fans know, and he might not even be a villain exactly, but he could be a great antagonist for a Batman on the lam.
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.