He could meet an experienced trainer who becomes his mentor. This mentor could teach Ash some advanced Pokémon training techniques that he hasn't known before.
Ash may come across a group of young and enthusiastic trainers. They might form a study group together, sharing their knowledge about different Pokémon types and how to catch them effectively. They could also exchange stories about their previous Pokémon adventures.
There could be a mysterious character in the trainer school. This person might have a special connection with a legendary Pokémon. Ash could be intrigued by this character and try to learn more about their story and the legendary Pokémon.
Ash might meet a strict teacher who has no idea about Pokémon at first but gradually becomes interested in them. He could also meet a rival in school who challenges him not only in academics but also in Pokémon battles.
Well, he may face new challenges in the trainer school. There could be tough exams on Pokémon knowledge and battle strategies. He might struggle with some theoretical parts but shine in the practical battle tests.
He could meet Leon, the undefeated champion of Galar. Ash would surely be excited to battle such a strong trainer and learn from him.
One possible fanfiction could be that Ash starts at the Pokemon Trainer School feeling a bit out of place. He's used to learning on the go in the wild. But at school, he meets new friends who teach him different strategies for battles. He might struggle with the classroom setting at first, like having to sit still and listen to lectures about Pokemon types. However, his real - world experience gives him an edge when it comes to practical training sessions.
Lillie is a likely character. She could befriend Ash and they might go on Pokémon adventures together. She has her own story and growth in the Alola region, and interacting with Ash could add more depth to both of their characters. Also, Ash might meet the Trial Captains like Mallow or Sophocles. They could challenge Ash in different ways and teach him about the island challenges.
He might meet some of his old friends again, like Brock or Misty. Maybe they will join him on his new journey through the regions.
Well, Ash might get into trouble for his unorthodox training methods. Since he's been self - taught for a long time, the school's rules and regulations could be a bit of a shock to him. For example, he might try to use a move on a Pokemon in class that is not allowed during school hours. This could lead to him having to do extra chores or detention, but also an opportunity to learn the proper way to train in a more controlled environment.
Gary could be one of his rivals. Gary has always been competitive with Ash. Another potential rival could be a new character introduced in the fanfiction, someone who is really good at Pokemon battles and wants to prove himself better than Ash.
He'll probably meet a lot of new friends who also love Pokémon. They might share different training techniques and tips with each other.
He might meet Gary again. They could have a friendly chat or even a Pokémon battle.