The timeline outside Harry Potter. I think Rowling should have given that period up to various authors to do their own thing under a single universe, like the old Star Wars EU. Imagine the possibilities, seeing Harry and Ron act as young adult aurors, Harry’s childern getting their own adventures in Hogwarts, maybe even playing up the 21st century tech into conflict with magic.
Instead, we got nothing, and after years of waiting, the one thing that “passed her approval” was a terrible, worse than the TROS in logic time-travel story.