My biggest problem with getting Depp to be in a Batman movie is that he's expensive now. Leger has had some big flicks, but he doesn't have anywhere near the draw that Depp does.
He'd make a very good Riddler because of his natural persona, and the same goes for Mr. Freeze. Freeze, for example, was a doctor and Depp could pull off a doctor type fairly easily. The biggest problem I see with both characters is his voice, but I'm sure he could manipulate it enough to make it work great, he's a very good actor.
The Riddler and Mr. Freeze aren't very big characters in the Batman universe, and it could pose a problem as this director seems to only want to use the very distinct characters in his work. No Robin, for example, even though the two are very closely tied together.