They've done their Joker movie for this series. Why do we need to see him again? Particularly when Ledger did such a brilliant, memorable job and when any other actor will inevitably be compared (and probably negatively) to him?
As for the 'We're destined ...' line - well, maybe the Joker got it wrong. He is insane, after all He's in Arkham, locked up and it looks like his prediction was wrong. I don't think too much emphasis needs to be placed on that line. And even if it was intended to be significant, well, we the audience all know what's happened in reality and he we won't see him again in this particular bat-continuity. There will be other 'we' like him, other freaks and crazies. And the Batman has as good a rogues' gallery as any superhero. Let's see them instead.