Maybe after the League, there can be an arc similar to the Looker sub quests in X/Y where you hear from Steven that his job as Champion doesn't allow him to spend much time looking for rare stones and look around Hoenn for his replacement, so he can go back to his hobby while still being a powerful Trainer.
This sounds like a nice idea. I would prefer something like this will Juan or Wallace, but either way, I like it.
Perhaps one of those two could....wait, have we not gone through this already?
Although a NPC that travels around the region that is related to Megastones sounds cool. Maybe we could have to find this NPC during a certain type of the day. Similar to X and Y's thing.
Of course something different would be quite refreshing.