I have always liked the idea of the Joker as something of a foil to Batman.
The idea that Bruce/Batman has the Joker in the back of his mind all the time, that the Joker represents a line that he must not cross has always fascinated me.
I envision in a 3rd (or maybe the 4th, give us the opportunity to forge about the guy) the batman gets pushed to this breaking point where he is seriously thinking about taking some lives, maybe he is coming to the conclusion that it is all that is left that he can do.
He goes into Arkham Asylum and and the gaurds sit the joker across a metal table from the Batman.
The joker, chained to the table can't shut up. Just babbling stuff, probably about how him and the Batman are one and the same, maybe about how the Batman needs him.
The Batman sits in stoic silence and never utters a word during the entire scene.
He gets up and leaves. The Joker freaks out and starts scraming...
I picture the joker/batman relationship as this permenant thing in the Batman universe.
I know that the movies will move on and we won't see the Joker again.
Such is life.