What with the "death" of Batman, I think that now would be a fine time to kill the Joker, too.
Have someone finally just kill the fucker. Or have him go catatonic or be put in a coma. Or see him just slink off into obscurity, utterly crushed by the fact that Bats isn't around.
Then we can see a World Without Joker for a while. Hell, maybe that can even provide some kind of meta-reason for why he's so invulnerable to death. Maybe he's protected by the Evil Clown Gods, I dunno. Maybe, Galactus-like, he serves a purpose so to speak. Perhaps in his absence a horde of lesser crazies would only spring up to fill the vacuum, making things worse than ever.
Anyway, when will there ever be a better opportunity to kill off the Joker for a while? Someone ought to go for it.