Satomine Night
The Power of Z!
I won't mind that because in that way, Ash will get the right Z-crystals for his Pokemon which would be nice. And gags are supposed to get old, you know. And though Tapu Koko is a guardian deity, it is fickle minded and it has an interest in Ash.
Tapu Koko doesn't need to intervene in order for Ash to get the right Z-Crystals for his Pokémon from the Trials.
Pikachu - Electrium-Z
Rockruff - Rockium-Z
Decidueye (if Rowlet fully evolves) - Grassium-Z; Ghostium-Z
Incineroar (if Litten fully evolves) - Firium-Z; Darkinium-Z
Everyone can use Normalium-Z, so that just leaves Groundium-Z, Dragonium-Z, and Waterium-Z (since Fightinium-Z is already out); and Ash can easily catch Pokémon who can use those Z-Crystals. The only question left is if and/or how Ash will get the Z-Crystals that are not earned through the Trials.
(There's also the question of what Z-Crystal will replace Electrium-Z when Ash completes the Hokulani Observatory Trial, but I have a feeling he will probably meet Molayne and end up with the Steelium-Z.)