Regardless of weather he redefined it or simply set a new standard, I do not think many directors will be able to duplicate his approach with much succsess.
And in a way, I don't want villains like the Joker becoming overused. just look at what happened after Dafoe's Green Goblin, suddenly everyone was using the insane corparate macihalovelianist angle, just look at Julian McMahon's Doctor Doom, that wasnt even his origional backstory, but they used it anyway.
Do we really want the same thing happening with the Joker? The last thing we want is for every villain to be a Joker rip off.
Sadly, I think this is exactly what is going to happen, movie studios are going to start seizing every ounce of creative control and start scrapping anything that isnt a Dark Knight rip-off.
Before you know it the comic book movie genre will be overpopulated with obviouse and balant TDK rip-offs, and Nolan will have to re-redefine the comic book movie genre all over again with his next movie.