Many leaks mention that the Night King's story is far from over and that Bran will travel to the past and learn about the purpose of the Night King. I am expecting a shocking twist in the final scene of the show: Either Bran becomes the new Night King, or the old Night King is reincarnated in a baby (maybe the one from season 4) and Tormund sees that. Such an ending would be a teaser for what's to come: The original Long Night prequel that features the Night King.
Or maybe DnD was just fucking with us Night King fans when he said that he couldn't tell if his story was finished in the show.