I think Depp could pull it off, but it wouldn't be the kind of surprising and shocking performance that we saw from Ledger.
Because we've seen so many of Depp's movies at this point, we can pretty much imagine what his Riddler would be like without ever seeing any footage.
Me, I'd much rather be surprised by the performance. I'd rather get a totally unconventional choice.
I remember back when they announced Ledger, I shook my head for days. "That guy? From the gay cowboy movie? That's gonna be the Joker? Is the whole world taking crazy pills?!"
And then the movie came out. And Ledger totally blew me away.
I guess the difference is ... in Tim Burton's Batman universe, actors were matched to their parts based on the public's perception of them. So if you asked back in the 1980s: who'd make a great Joker ... almost everybody would have said Nicholson. Same with DeVito as the Penguin or Pfeiffer as Cat Woman.
So if we were still in Burton's Batman universe, Depp would be the most obvious choice for the Riddler. So obvious, in fact, some might even call it "stunt casting."
But Nolan seems to be after something different. Which I really dig. He's casting the best actors he can get. Not the biggest movie stars. And he's asking them to stretch beyond their comfort zones and surprise us with performances we haven't seen before.
Personally, I'd love to see a complete unknown take the reigns as the next villain. As long as they're a great actor, I'm all for it!