Sort of like how Dragon Ball Z overtook the source material while the manga was being written, witth filler arcs in the 1990s and 2000s anime?
I don't think ther'd be filler arcs, but probably stretch out interactions more or perhaps make some episodes focus on the side characters. Wouldn't require much input from Rowling outside of general approval of their characterization and keeping in mind that it is set in England of the 1990s.