There are seemingly 7 identical plots of land scattered around Alola.
There are 7 trial captains and 4 kahunas, leaving 7 types and their corresponding Z-Crystals seemingly ignored.
Silvally, Cosmog, Lunala, Solgaleo and Zygarde don't feature in the Alola Pokédex, indicating they can't be caught in the main game.

It seems that the Island Trials are a sort of "prologue" to the Pokémon league to me.

Part 1:
>Do the trials
>Beat the 4 kahunas
>Defeat Team Skull

Part 2:
>Bracelet upgrade allows for use of megas
>Beat gyms
>Collect the Tapus in their hidden temples
>Fight Aether Foundation
>Pokédex upgrade allows for non-Alola Pokédex Pokémon to be found in hidden areas (maybe some cheeky new Alola forms like Ekans)
>Solgaleo/Lunala climax
>Elite 4

Part 3 - actual post-game
>Receive Silvally from Gladeon
>Beat Red and Blue
>Re-match the Elite 4 and/or Kahunas
>Battle Tree