I assume there aren't many (not necessarily young) people left who haven't seen The Dark Knight Rises yet. OK, there might be some who still refuse to watch it - but I'm definitely not talking about those.
Some fans were disappointed (long before the movie started) that Christopher Nolan didn't replace Heath Ledger's role as the Joker (may he still rest in peace) with another actor. I have to admit that I am truly happy that he didn't even try. Heath did an awesome job on screen and I guess that everybody would have compared any new actor with him - and not in the new actor's favor. So we've got Bane and are also reminded of Ra's al Ghul and Scarecrow. Oh, and we got Catwoman (and I love Anne Hathaway, she's just so beautiful and alluring), though we can't really perceive her as a villain. Yet she's there and we can enjoy her as well as the other guys who try to get in Batman's way.
I think that The Dark Knight Rises works pretty well without the Joker being a part of it. After all, we all know that the Joker has obviously been caught at the end of The Dark Knight, when Batman leaves him hanging for the police to collect him. We can assume that the Joker is stuck in Arkham, probably bored and with nothing else on his mind than escaping the asylum. We also know that he might come back sooner or later - which, of course, doesn't apply for Nolan's work as he doesn't want to continue with this story.
Regarding the many comics and graphic novels in which the Joker appears, he and Batman depend on each other - whether they like it or not. And in my humble opinion, Batman and the Joker are two of a kind. Yes, the Joker kills and Batman won't unless he absolutely has to. Yes, the Joker is insane - but Batman's mental ... condition is at stake too, and more than once so. They belong together, really. Probably they should just meet at a cozy bar and have a drink. Talk about life in general and Gotham in particular. If they hung out on a regular basis, they might be able to get along pretty well.
I am absolutely serious about this. They are "one and the same, two of a kind" (Incubus, Neither of us can see), even though they pursue different goals (if any at all, considering the Joker). And I'm pretty sure that we will see them together again on the big screen. If anybody out there finds an actor who can manage portraying him well. And in case we get a new Batman, that is. Until then, we can have fun with Nolan's work and enjoy Batman Begins, The Dark Knight, and The Dark Knight Rises all over again. Drooling included.
Goodevening.