Actually, TDK has set the bar so high that it would be almost impossible for a sequel to meet our expectations. Joker has always been the only villain who mattered in the Batman universe and his absence would surely not go unnoticed. Tom Hardy is a fine actor but it would be a great challenge even for him to fill the shoes of the great Heath Ledger,not only because ledger has received a cult status but also because the character of Bane is overall much weaker in substance compared to the Joker.
That is not to say that TDK Rises wouldn't be a great movie. Nolan is an exceptional director who brings out the best in his actors. I remember, before TDK was released, everyone including me were very much spectacle about Ledger as joker and his makeup in the film. But we all know how that turned out. Here's hoping that the series ends on a high note.