Got an idea. What if on the way to Kalos or Alola the ship bumps into an unmarked island during a storm and the MC decides to explore the island while the rotom repair any damages the boat suffered. While exploring he finds the Arceus phone from Legends Arceus. When he picks up the phone he is instantly teleported. Then he meets the original Arceus and Arceus asks him to travel back to the past to impregnate the hisuian pokemon in past Sinnoh(aka Hisui) because Arceus wanted to save the pokemon, specifically their hisuian forms, because back then they were starting to go extinct. This will make the game have an additional reigon, Hisui. Once that is complete the MC returns to the future with no trace of the Arceus phone left, like he never left in the first place.
It's a big ask so you might not do it and that's ok.