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I'm starting to believe that Lillie is actually going to be the main pokegirl who travels with Ash later on in SM.
We're almost half a year into SM and with the way the series has gone so far we've learned one important thing about the cast, which is that they all have a life and family outside of the Pokemon School. Lana takes care of her sisters, Mallow works at her families cafe, Kaki works on his farm and does deliveries throughout the Alolan Islands.
Lillie though, she doesn't do anything afterwards as far as we can tell besides return home with Shiron.
If we do get a "summer vacation" arc like I'm expecting, it's quite likely that Ash is going to use that time to start travelling the rest of Alola to explore, fight captains, do trials, and defeat Kahunas.
This gives Lillie an opportunity to do something with her time, which could lead us to a path closer to the games which has Ash, Lillie, and Kukui travelling through the Islands together for the most part.
This allows us to have excitement and hype for when the cast we've met over the last 20 episodes returns every so often for an episode or two leading to their trials. It also would lead to Gladions introduction, Aether and Skull development, but more importantly a chance at Lillie working on her fear of Pokemon.
The Pokemon School is a safespace for her, she's going to need a dose of the outside world before she can fix her fear.