In the latest episode, Kukui hinted he may plan to establish the Alola League, similar to how he did in the games. If Ash reaches the League with only the current four Pokemon of his Alola Team, he could potentially bring back some of his older Pokemon like he did in the Sinnoh League.
I mean, who'd love to see some of Ash's very forgotten Unova team get a chance to shine?
If there is Pokemon league in Alola then the Gyms will also return right?