Death is rarely forever in comics anyway, and I really doubt that the Joker will be killed off forever. Assuming that is even what Morrison is doing. The real deal is that they always comes back.
I think it would be pretty hard to forever kill the Joker off. Post-COIE he became much more than just another one of Batman's kooky villians. He went from being a zany prankster to a truly frightening, completely insane and unpredictable stone cold killer - even more lethal than when he was first written back in the day.
The upshot of this, as I see it, is that he became Batman's true nemesis. Ra's and Bane might appear to be Batman's opposite numbers in that they are very much LIKE Batman but on the opposite side of the fence. But the Joker, in his modern incarnation, he is everything Batman is not. Bane and especially Ra's operate with a kind of sense of honor, as twisted as it might be. They function within a set of rules they define for themselves and ulimately (like Batman) appear to have lines they will not or cannot cross.
The Joker, however, has none of these things. Batman has precious few lines he won't cross, but he has them. The Joker has no lines, appears to have no motive, no reasons for doing what he does. To taunt Batman? Why? Because he enjoys killing? Perhaps, but why dress it up in the garish clown scenario? The only person who understands why the Joker does what he does, is the Joker himself. And he might not even know. Never forget that line in the Killing Joke: "Sometimes I remember it one way, sometimes another. If I have to have a past, I prefer it to be a multiple choice."
If Batman is a respresentation of the struggle of order against chaos, the Joker is like the Norse god Loki. Chaos personified. He might sometimes actually BE funny. He says a couple of good ones in the Killing Joke. Or he is just terrifying like in DKR, but at the end of the day he isn't like Riddler, leaving clues for Batman to follow. Some will argue that the silver age Joker was testing Batman's intellect by giving him clues, but that verison of the Joker wasn't dangerous. Testing Batman's mind didn't make the Joker Batman's real rival. It's the utter, lethal and complete mahem that he inflicts.
Once you add the personal grief the Joker has unleashed on Batman - murdering Gordon's wife, murdering Jason Todd, shooting Barbara, etc, you have a character that is linked forever, perhaps in a sick way, to Batman. And their relationship appears to be the most fatal kind. It is often implied they are fated, at some point, to destroy each other. Like Holmes and Moriarty they are doomed to go over the falls together. (That is why I always thought Miller got the Joker bang on in DKR..without his "other half" the Joker falls into a near catatonic state, only to come back to life on the heels of Batman's return).
Any "new" Joker will simply not be this sort of fundamental opposite to Batman. If, and lets face it at this point it is a big IF, Morrison is creating some kind of new Joker, the challenge will be to create a different kind of Joker. Whatever form this character takes, it won't take the same kind of toll out of Batman that the "classic" Joker does. He might be interesting to read, if Morrison is doing it then you know it will at least be unique, but it won't tap into the same kind of twisted and fatal Rota Fortuna that the "real" Joker and Batman are locked in. If it did, it wouldn't be a "new" Joker.
Ultimately, I think it will be very hard for a "new" Joker to be anywhere near as compelling. In one of the Batman documentaries on the Tim Burton Batman DVDs Kevin Smith put it best, IMO. He said that like Batman, the Joker begins with some kind of devastating tragedy. But where Batman dedicates himself to trying to impose order so no one else has to suffer like he did, the Joker has decided to impose chaos so that EVERYONE suffers like he has.
It's going to be pretty hard to top that, and that is why even if the Joker is "killed", he'll be back.
