Well, perhaps it could be about a magical event that causes Ash to transform into a baby. Then his friends have to take care of him while also trying to find a way to reverse the transformation. Maybe Pikachu has to protect baby Ash from other Pokémon who might mistake him for an easy target.
Obviously, Ash would be a main character even in his baby form. Pikachu is bound to be a key character as well, being Ash's closest Pokémon partner. His other Pokémon like Bulbasaur, Charmander, and Squirtle could also play important roles in protecting and taking care of baby Ash.
In an 'Ash turns into Latios' fanfiction, perhaps Ash is turned into Latios as a way to better understand the Pokémon he has befriended. As Latios, he might be able to communicate with other Pokémon on a deeper level. For example, he could learn the secrets of legendary Pokémon that have always been a mystery. His friends, like Pikachu and Misty, would initially be shocked but then have to figure out how to help him reverse the transformation or support him in this new form.
One possible story could be that Ash is hit by a strange Pokémon attack that turns him into a Pichu. As a Pichu, he has to rely on his Pokémon friends in a whole new way. Pikachu, for example, might take on a protective role, guiding Ash - now a Pichu - through the challenges they face. And Ash would have to learn to control his new Pichu abilities like using Thunder Shock in a more instinctive way since he's now in a Pokémon's body.
In a fanfiction where Ash turns into a Vulpix, it could start with Ash exploring an ancient and forbidden area. There, he disturbs an old relic or power source that causes the transformation. As a Vulpix, he discovers that he has new abilities such as being able to use Ember and Quick Attack in a more instinctive way. His friends, like Misty and Brock, would be shocked but would quickly try to find a way to reverse the transformation. Meanwhile, Ash in his Vulpix form might make new Pokémon friends who are initially wary but then accept him as one of their own, leading to some interesting adventures in the Pokémon world.
One idea could be that in this fanfiction, Wolverine turns into a baby due to a powerful mutant's accidental spell. The X - Men then have to take care of him while also trying to find a way to reverse the transformation. They might discover new aspects of Wolverine's personality as a baby, like his unwavering determination is now shown through his attempts to crawl towards danger.
Another possibility is Ash becoming Lugia. Lugia is known as the guardian of the seas. As Lugia, Ash could explore the deep oceans, discover new species of Pokemon, and safeguard the marine ecosystem. He could use his psychic and flying abilities to interact with both water - and flying - type Pokemon, bringing a new era of harmony between different Pokemon types.
Well, if Ash were to turn into Mew in a fanfiction, it would be quite a wild turn of events. Mew is known for its ability to transform and its vast psychic capabilities. Ash, with his new form, could potentially travel to different dimensions that were previously inaccessible. He could also communicate with Pokémon in a whole new way, using Mew's psychic powers to understand their deepest thoughts and feelings.
Well, if Ash turns into a Pikachu in a fanfiction, it would be quite a transformation. Firstly, his communication with Pikachu would become more of an internal dialogue. And he'd have to get used to moving around on all fours and using Pikachu - like movements. His battles would be completely different too. He'd be relying on Pikachu's natural skills rather than his own battle strategies as a human trainer.
In fanfiction where Ash turns into a girl, it could lead to some interesting plot twists. Maybe his relationships with his Pokémon would change in unique ways. For example, some Pokémon might be more protective of him now that he's a girl. Also, his interactions with other trainers, like Misty and Brock, would be different. They might treat him with more chivalry or have different emotional responses to his new form.