Joker casts a pretty big shadow for everyone else to be living in. I think its safe to assume that the reason TDK is so big is Joker alone. The movie could be about him alone and the hype would still be huge.
The one thing I hope Nolan and co. are careful of, is pairing villains off with each other. That was part of what ruined BF and B&R for me.
It's also what made Returns so cool. Batman is facing two villains acting independently of each other. So instead of going to The Riddler's lair and stopping both villains, he takes care of one and still has to deal with the other. Think the first chapters of Knightfall, leading up to the final fight with Bane. Just threat after threat after threat.
Now what I don't mind, is having one villain leading all the others. In this case, Two-Face leading the other freaks. Multiple villains "teaming up" to topple their common foe is different than two working together.
All in all, I think the 3rd could easily topple the second, provided they give the fans what we want, without turning it into Spider-Man 3.
To tell you the truth, I wouldn't even mind if the 3rd ended with a cliffhanger or hint of what's to come, and they never made another film.