As I said earlier, Ash should beat Diantha, because his first two battles was interrupted, and in third battle should luck. He became a champion, next - filler episode in Sycomure's laboratory with two new trainers (boy and girl) and choice with starter Pokemon. Also in there, or later, episode with Braxien evolution. And then Ash with friends come in Kanto for celebrating his victory, and 11th birthday. And this episode should be very Special with all main characters from previous seasons, and Cassidy and Butch. After party, Serena decide back in Kalos for her dream, Deddene like Piplup in final of DP, and in the Ash comes with Serena, for help in fulfill her dream, in gratitude for her help to fulfill his dream. The end of Kalos saga and Ash's story. Beautiful, emotional and epic! And Beginning of story a new main character in Alola.
https://www.youtube.com/watch?v=UxupxTrv-90
https://www.youtube.com/watch?v=tvum-dDCLY0