Sun and Moon: World Championship Theory:

So Alola was partially chosen to be Hawaii because the Pokemon World Championships are often held there, and we know Sun and Moon wont have traditional gyms or a league.
Why does Alola decide to form a leauge now?

Maybe because the Pokemon World Championships are taking place in Alola that year in game. They've even been referenced in game before like an NPC in the Battle Resort saying he cant wait to compete in the next one.

It could be like the PWT but expanded to include elite 4, Frontier Brains, Notable trainers, (Wally,Zinnia,Eusine,etc.) and maybe even other playable characters.
Having all these notable characters come together for a massive tournament and vacation would be a great tie in to the 20th anniversary and be how it "all comes together"

I think the Artificial Island will be the spot where the World Championships take place. We had the backpacker in XY say his region "had a facility you wouldnt belive"
Maybe he was referring to the Island, a massive resort where the trainers stay and the tournament is held. It could be like a mix of the Battle Frontier with several facilities and the PWT where you compete with the old characters, along with a hub area like Mauville City where you can find shops with move tutors and such.

Perhaps the player character was chosen as Alola's representatives for the tournament since they were chosen by Tapu Koko in the beginning of the game, and now they have to become the Island Champion to prove their worth, by completing the trails, earning the blessings of Solgaleo and Lunala, taking down the troublesome Team Skull, as well as scout the most powerful trainers to represent Alola as the new gym leaders and elite 4

Whaddaya think?