Perhaps Ash turns into a fairy as part of a prophecy. The fairy form gives him a new perspective on the Pokémon world. He realizes that there are more hidden secrets and powers. He uses his new fairy abilities to help his friends in battles in unexpected ways. For example, he can now use fairy - magic - based attacks that are different from his normal Pokémon attacks. His transformation also makes him a target for some villains who want to use his new powers for their own evil purposes.
The plot might involve Ash being cursed to turn into a fairy. He then has to go on a journey to find a way to reverse it. Along the way, he meets other fairy - type Pokémon and fairies who either help or hinder him. He could learn about the fairy world's culture and traditions, and this knowledge could be the key to breaking the curse. Maybe he has to prove himself worthy in some way, like by helping the fairies solve a big problem in their realm.
One possible plot could be that Ash, as a female, has to face different challenges in the Pokémon world. For example, some trainers might underestimate her at first. But she uses her knowledge and skills to prove them wrong. She might also form different relationships with her Pokémon. Maybe her Pikachu would be even more protective of her in this new form. And she could have unique adventures that are different from her male self's, like being more involved in Pokémon contests due to the perception that female trainers are more into that aspect of the Pokémon world.
One possible plot could be that Ash wakes up one day as a girl and has to deal with all the changes in his daily Pokémon adventures. For example, he might have to adapt to new clothing styles and different social interactions. His Pokémon might also react in various ways to his new form.
One possible plot could be that Ash is turned into a Pikachu by a strange spell cast by a mischievous Pokémon. As a Pikachu, he has to figure out how to communicate with his friends. His Pokémon pals at first don't recognize him, but Pikachu - Ash starts using his old battle strategies in a new way. For example, he tries to direct Pikachu's electric attacks in the same patterns he used to direct his other Pokémon's moves in battles. Eventually, his friends realize it's him and they set out to find a way to reverse the spell.
One idea could be that a powerful magic spell backfires on Ash. His friends then have to take care of baby Ash while trying to find a way to reverse the spell. They might face challenges like getting baby Ash to eat Pokémon food or stop him from toddling towards dangerous Pokémon.
Well, he could have all sorts of wild adventures that deviate from the original story. Maybe he'd be a different type of Pokémon trainer, like one who specializes in psychic Pokémon instead of his usual variety. He could also form new relationships with other characters, both human and Pokémon, in ways that the original anime didn't explore.
Some of his Pokémon like Pikachu might be very protective. Pikachu would stay close to Ash at all times, being extra vigilant in case any danger comes while Ash is in this new form. It would also be curious and might try to play with Ash's new fairy features, like his wings if he has any.
Well, he might start to see the world from a Pikachu's perspective. For example, he would experience the small size and how everything looks so huge. He could also find it difficult to communicate with his human friends in the normal way since he is now a Pikachu. Maybe he would have to rely on Pikachu's ways of expressing emotions like using electrical signals.
Perhaps in this fanfiction, Ash discovers that he has a deeper connection to Mew than he ever thought. It could involve Mew choosing Ash as a special trainer to carry out a very important mission in the Pokémon world, such as saving a certain region from a new and powerful threat. This would lead Ash to face many challenges and grow in ways he hasn't before.
The way Ash adapts to Pikachu's instincts could be really interesting. He might find himself being more drawn to shiny objects or having an urge to store food in a certain way. And his relationship with his own Pikachu (if it still exists in the story) would be very different. They could become like equal partners instead of trainer - Pokémon.
One possible plot could be that Draco is turned into a kid by a misfired spell. His friends, who are used to his haughty self, now have to deal with a cute, innocent, and sometimes clueless kid - Draco. They might start to see a different side of him and form new bonds as they try to figure out how to turn him back.