I think Hugo Strange or Black Mask would be villains that really match Nolan's style and feel.
I don't remember where I saw this, but if I recall correctly, I believe I read an article a while back - an interview with Chris Nolan, in which he discusses the villain. You know how random the villain was in "Batman Begins"? Nobody would've ever guessed that. Well, he basically said that the villain in the next movie would also be a lesser-known villain from the Dark Knight universe. So, I wouldn't hold my breath for the Riddler.
Ra's Al Ghul and The Scarecrow? Are they really lesser-known villains?
I had never heard of the Scarecrow, before the movie. I have a couple Batman comics, including a giant sized "Batman versus Manbat!" I guess Manbat isn't well known, either.
Catwoman! Catwoman! Catwoman!
Well, the thing is, (and correct me if I'm wrong, Clapper), I believe these movies aren't based on the Batman character in general, but on a specific series of comic books -- The Dark Knight comics, or something like that. So, there's a specific storyline, and I assume (based on Clapper's post) that in this storyline, I guess the two villains from "Batman Begins" are pretty huge in those specific comics.
It IS Nolan's last Batman in the franchise, he mentioned.
You know who should take it over? Darren Aronofsky.
I go poopy in my pants. My friends call me Poopy-Pants. Sometimes, I even poop out of my mouth.