I don't really know if there should be a sequel or not. It would be kind of hard to top this movie and make another installment.
I also hope that the Joker does not return if there is another movie. Heath Ledger did such a great job in this movie that I don't think it should be attempted to be duplicated. I don't know if the reason the Joker wasn't killed was to pay tribute to Ledger by keeping his character alive or to use him again later, but I hope the latter isn't the case.
BTW, for those of us who did not catch on, what were the hints of Catwoman and Riddler?