Maybe if Nolan decides against recasting the Joker, his role can be filled by Harley Quinn in the 3rd movie. It would be a shame to not have them on screen together, but it would be an even bigger shame to not have any continuation whatsoever of Joker's presence in the 3rd movie. Especially since indications are that they were planning on bringing him back.
Say Dr. Quinzel fell in love with Joker while treating him in Arkham, but he gets the death penalty (I'm assuming Nolan didn't kill in TDK). She snaps when he dies, carves herself a smile, and carries on his legacy. This can all have happened between the events of the 2nd and 3rd movie, so that maybe Batman has to figure out who this crazy clown lady even is. This way Joker would really be an absolute, in that his existence is perpetuated even beyond his death by Harley.
I'd love to see Kristen Bell in the role, but if Harley was going to have to go solo, then they should go with a high caliber actress, say Rachel Weisz (or someone along those lines), who'd really lose herself in the role and give just as intricate a performance as Ledger seems to have given.
Hmm I know I just said high caliber actress, but it just occurred to me that Milla Jovovich might very well kick some serious ass as Harley. She tends to be in bad movies (don't understand why she kept going for those crappy Resident Evils) but she is actually quite good if you've seen some of her performances, and can play crazy easily. I could see her pull a Ledger with Harley.