Your point was that they couldn't link Nolan's films to the new ones. You just didn't express it very well. And Ledger is not the key to continuing that universe. It's irrelevant. Yes, we know Nolan wouldn't recast the Joker, but that universe could continue without that character. Opinions aren't facts. Just because you think Nolan threw a story together on the fly, doesn't make it true. Just because you think we didn't get closure from "its most interesting character/dangling plot thread" aka The Joker, doesn't mean that's true or that they couldn't continue that universe.
It's not realistic to think Bale would return, i get that. But the most realistic option is for Bale to play a whole new version of the character, with a new Joker (if Reeves wants to include him).