In this kind of fanfiction, perhaps Ash loses a big Pokemon battle and his Pokemon feel disappointed and abandon him. This could lead to Ash going on a journey of self - discovery to regain their trust and friendship.
It could be a powerful and jealous trainer who wants to take Ash's place. This trainer might use dark magic or some underhanded tactics to make Ash's Pokemon abandon him so that he can swoop in and claim them for his own.
One common theme is maternal love. Ash's mom is always shown to care deeply for him. Another theme is the home - a place Ash returns to and where his mom provides comfort. Also, there's the theme of support, as his mom is often depicted as being his biggest supporter in his pokemon - catching and battling adventures.
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.
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.
The fanfiction might also explore Ash's inner turmoil. He could struggle with self - doubt, thinking that he was not a good enough friend. However, as he faces various challenges on his own, he realizes his own worth and that the abandonment was more about his friends' own issues rather than his shortcomings.
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.
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.
In some fanfictions, Ash gets angry. He might be angry at the world, at fate, for taking his mom away. This anger could manifest in different ways, like being more reckless in battles or snapping at his friends when they try to comfort him. But deep down, it's just his way of dealing with the overwhelming pain of losing his mother.