Imagine this: Necrozma steals the light in our world and at the same time swarms of Ultra Beasts attack Alola region, even more than in SM

The player needs to go after Necrozma with Ultra Recon Squad while Tapus and Kahuna's handle the UB invasion but its too much for them because the invasion is on a much grander scale

Looker and his people call for champions from other regions (they wanted help from a champion in SM) and they come as fast as they can using flying and psychic pokemon
Lance helps Kukui and Hala on MeleMele
Steven and Wallace help Olivia on Akala
Red and Blue help to protect the Aether Foundtaion
Alder and Iris help Nanu on UlaUla
Cynthia and Diantha help Hapu on Poni

In the post game all the champions decide to stay for the Battle Tree challenge (or even better: Alola hosts another PWT) and they're the title defense challengers

Wouldnt that be cool?