The Joker is Batman's ultimate opponent, his nemesis, his exact opposite. The Joker was meant to be in the sequel to TDK as I specifically remember interviews prior to the films release with Nolan and co discussing it. Since, I've been completley unable to locate them, probably because of his death. Of this I swear, I know many other people remember them also.
Bane may be intellectually and physically superior to The Joker, but The Joker is undefeatable (this is definitely not a word, and if it is, that's not how you spell it
) , that's the point. You can't stop him. He's just.. chaos. He understand Batman's psyche and exploited it better than any other opponent likely can. In this context, it's impossible to top this character.
That said, I don't think TDKR will try to top The Joker. I do, however, believe Bane, and the story itself will be stronger and fiercer than TDK's, and that is what will raise the stakes to what they need to be.
Lastly, The Joker is probably 70% Heath, 20% writing, 10% makeup/costume. Chris gave Heath almost total creative control of the role, and Heath just... there's a reason nobody was uncomfortable watching him on screen despite his death. We weren't watch Heath perform, we were watching -THE- Joker.
I dunno why everyone is saying Hardy can top Heath's mind-blowing performance, I certainly haven't seen him in anything that's even approaching that caliber of acting, but I do know I hope you guys are right, I just don't see how you all could be.
The story is what will make TDKR a better film, not the villain. I have a feeling TDKR' will deliver the most unconventional story yet. That's what will deliver.