Well, he might start to see the world from a Pokemon's perspective. For example, he could understand their communication better and form closer bonds with them. He may also face new challenges like having to adapt to living in a Pokeball or dealing with the natural instincts of his new Pokemon form.
In many fanfictions, Ash achieving the status of a Pokemon Master often involves him having extraordinary battles. He might defeat powerful legendary Pokemon that no one has ever tamed before. His Pikachu could evolve into a more powerful form, which gives him an edge in battles. Also, his bond with all his Pokemon, not just Pikachu, becomes so strong that they can execute complex strategies without any communication barriers.
In some Pokemon fanfictions, there could be creative interpretations where Ash is made out to be Red's father. However, in the official Pokemon canon, there is no such relationship between them.
One possible plot could be that a spell or a magical artifact causes Ash to turn into a girl. Ericka might be involved in the discovery of this transformation. Ash then has to deal with the new physical form while still trying to pursue his Pokemon journey. His Pokemon might initially be confused but still recognize him. And he could face different challenges and reactions from his friends and rivals as a girl.
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.
It could be that there's a prophecy or a mysterious force at work in Erika's garden. Ash, being his curious self, stumbles upon it and it transforms him. The fanfiction might explore how his Pokémon react to his new form. Some might be confused, while others could be protective. Ash, as a girl, would have to face challenges like different expectations in Pokémon battles and dealing with rivals who see him as an easy target now.
In this fanfiction, if Ash turns into a pokemon, perhaps he becomes a Charmander. He could struggle with being in a different form. He might have to adapt to the life of a fire - type pokemon, facing challenges like being weak against water - type attacks and trying to find a way to change back while still having adventures with his friends, who now have to protect him in a different way.