There could be a new character, a rival to Ash's dad from his younger days when he was an active Pokemon trainer. This rival could reappear in the fanfiction, causing some trouble or adding some drama to the story.
Perhaps in this fanfiction, Ash's dad was once a great Pokemon trainer but had to give it up for some reason. Now, as a fan, he lives vicariously through Ash. He could be the one who inspired Ash to start his journey in the first place. He might have told Ash all those amazing stories about Pokemon when Ash was a little kid, which planted the seed for Ash's love for Pokemon.
If we consider the common growth pattern in the Pokemon series, Charmander evolving into Charmeleon could be an early evolution in the fanfiction. Since Charmander has a clear evolution line and Ash has a strong bond with it, it would make sense for it to be one of the first to evolve, perhaps due to a tough battle or a special encounter that accelerates its growth.
One idea could be that Ash discovers a Sylveon in a forest. The Sylveon is injured and Ash takes it upon himself to nurse it back to health. This could lead to a strong bond between them, and Sylveon could become a unique addition to his Pokémon team, with new adventures and battles ensuing.
In fanfiction, Ash might improve his pokemon's IV through special training regimens. For example, he could focus on specific battle scenarios that target areas related to IV stats like speed or strength. He might also seek out rare items in the fanfiction world that have the power to enhance these inherent values.
Obviously, Ash is one of the main characters. He's the protagonist who has a passion for pokemon and is always eager to become a pokemon master. And the legendary pokemon he encounters or befriends would also be important characters. For instance, if it's Lugia, Lugia would play a major role in the story as it's a very powerful and mysterious pokemon.
His Pokemon might be a bit confused at first. Pikachu, for example, is used to their adventures on the road. But they'll soon adapt. They could make new Pokemon friends at the school too. And they'll be there to support Ash during his school challenges, like in battles or tests.
I'm not entirely sure as I haven't read that specific fanfiction. But generally, it might be about Ash's adventures with his Pokémon in a rather unique or different way compared to the original Pokémon story. It could involve new challenges, relationships, or character developments for Ash and his Pokémon.
Obviously, the Legendary Pokemon would be a main character. It could be a Pokemon like Mewtwo, who is often depicted as having a complex personality. It might start off as a mysterious and powerful entity that Ash has to befriend or battle against.