Question: what to do after completing pokemon sun?
Answer: Get Incinium Z, Decidium Z, or Primarium Z from Hau.
Get a Lucky Egg and TM90 Substitute from Professor Kukui in the Pokémon Research Lab.
Catch the four Tapu.
Catch the Ultra Beasts.
Go to the Battle Tree.
Get Cosmog.
Get Type: Null.