There could be a Pokemon that is a cross - breed or a genetic experiment at the school. Ash might be the first to discover its unique abilities. Another possibility is that he encounters a Pokemon that has a special bond with the school grounds, like a guardian Pokemon that he has to befriend in order to access a hidden part of the school.
One possibility is that Ash would be a bit of a star at the Pokemon school. Since he has so much experience traveling and battling. He could inspire other students to be more adventurous with their Pokemon. Also, there could be some unique Pokemon species that are only studied at the school which Ash gets to interact with.
One possible plot could be Ash starting high school in a new region where Pokemon battles are a major part of the school curriculum. He could meet new friends who have different Pokemon and battling styles, and they form a team to compete in the school's Pokemon battle tournament.
They could participate in a high school Pokemon tournament. Ash would be super competitive, using his Pikachu to its full potential. May might enter with one of her cute Pokemon like Skitty. They could face off against each other in an intense battle, which would be a really exciting part of the fanfic.
Well, in such a fanfiction, Ash and May could go on an adventure to catch a rare Pokemon. They might combine their skills, with Ash's battle experience and May's Contest knowledge.
The plot could be about a big Pokemon tournament where Ash and Tyson meet in the finals. It would be a high - stakes battle with both trainers using their best strategies and Pokemon.
Well, in this fanfiction, Ash might face new challenges at school. He could meet other trainers - in - training who have different views on Pokemon. Maybe there would be classes about Pokemon history and types, which Ash would either ace or struggle with, considering his hands - on experience in the wild.
Gary could be a main antagonist. He might be jealous of Ash's popularity at school and constantly try to undermine him in Pokemon battles and in front of their classmates.
Another possibility is a Celebi. Celebi is a time - traveling pokemon. Ash could be led to Celebi by some strange time - related events. It would be a challenging catch as Celebi can manipulate time to avoid capture. But with Ash's experience and the unique skills of his pokemon, he might be able to succeed.
In a fanfic, it could be that Ditto is a key to unlocking a hidden mystery in the Pokemon world. Ash, being the curious and adventurous trainer he is, teams up with Ditto to explore this mystery. They might travel to unknown regions, meet new Pokemon species, and have to battle against powerful and unknown foes along the way. As they progress, Ash learns more about Ditto's true nature and how it relates to the overall mystery.
In a Pokemon fanfic where Ash trades Pokemon, a lot of interesting things can occur. He might trade a Pokemon that has been with him for a long time, like Pikachu. This could lead to a whole new adventure as he gets to know the new Pokemon he received. It could also change his battle strategies.