We had the same discussion about a third movie. Sentiment was mixed. I think they could successfully pull it off, but they've upped the ante on the villain. They would need a home run of an actor to play a home run of a role. A dark, sinister Riddler, for example, could be pretty spectacular if done right. They've also set it up pretty well, though I also felt like you could conclude the franchise when this movie ended.
I don't think I agree that a third movie would necessarily ruin this iteration of the franchise. These are fantastic movies - a third wouldn't really impact them at all, in my mind. The Dark Knight stands fairly independent of Batman Begins.
We also started asking - is this the best action movie ever made? I think it might be. Certainly on the short list. Definitely the best Batman movie, almost definitely the best comic book movie...