I think The Riddler is an underrated character, even in the comics and animated series. I want to see The Riddler played out like he was in the story "Dark Knight, Dark City." It's probably the best arc the Riddler has in the comics. If Nolan uses The Riddler, he'll write him as a serious, cerebral character...which will automatically elevate it above the campy Carrey portrayal, even before Nolan casts an actor.
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.
Honestly, if you're a director, even Nolan, and Depp says he wants the role, you gonna actually tell him, "no, I don't think you're right for this?"
Well, I think that's not the case. I don't think it's a matter of "are you talented enough to play Riddler?" Depp clearly has talent. I think it's more of "do you fit the vision I have of the character?" Whether Depp does or not is all up to Nolan. Despite Depp saying he's interested in the role. God knows how many talented actors have expressed interest in roles, but have never been considered or cast.