Well, The Riddler IS more than just a thief. He's killed people. Seriously, for a good Riddler story, read "Dark Knight, Dark City." You can find it in Batman #452-454. Here is what I posted in the characterization thread:
He's been known to engage in grand theft, assault, attempted murder, and murder. But he generally can't just shoot someone and be done with it. He has to concoct some intricate death trap and either - generally, if the victim is Batman - challenge the victim to find the one and only way to escape (yes, Jigsaw from the Saw series is, in some ways, a major Riddler rip-off) or - if it's someone other than Batman - provide Batman with riddles that he had to unravel in order to save the intended victim.
And of course, that's his M.O., no matter what kind of crime he's committing. Any crime that can described in a riddle, The Riddler can, and will, commit. And if they do another film version of The Riddler, they really need to make his riddles difficult, which they've never done in previous on-screen representations. The Riddler's riddles should require genius-level intelligence and problem-solving skills to solve. That's the point. Riddler is a narcissist who has to constantly prove to the world that he's smarter than everyone. And Batman can only thwart him by being even smarter.
I always felt this could be where Reese would come into play. I always thought that the Riddler should kidnap Reese, make him spill who he is accusing of being Batman (which it ends up being true), and then putting him through one of his games for Batman to find or something. I think it would be better to kill him off, just so he can't be used again to spill the beans. This could be the point where Batman realizes that the Riddler is a bigger threat than he initially thought. Where it's much more than just riddles he needs to solve. Coleman Reese could be his first victim or something.
That's why I think he'd be an interesting villain to go with for Batman 3. I also think Black Mask should be in there too. However, rather than having him be someone inspired by Joker, I'd rather have him be one of the many patients that escaped from Arkham in Batman Begins. I don't think any villain should be inspired by the Joker (except if they go Harley Quinn) in any upcoming sequel.