I think you give too much credit to Heath. True, he was shining in this role but you know there are other great actors out there who can also do an amazing job with a proper script.
So don't say other villains can't be as good or better than the Joker!
About closure: there are some ways they could do this. For example don't explain anything, maybe say a few words about it, just leave his dissapearance a mistery for some time and maybe a few films later a new actor can jump in to continue or finish that story. I'd like this one a lot.
Oh, and I'd love to see Uma again too, she is so hot!
