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.
He could find that his relationship with his Pokemon changes. Some Pokemon might be more protective of him now that he's in a more vulnerable form. Others might see him as less of a trainer and more of an equal. Ash would also have to deal with how the Pokemon world views hedgehogs. Are they seen as pests or something more interesting? This could lead to different reactions from different Pokemon species. And as he travels, he might discover new places that are more suitable for a hedgehog - like small burrows or areas with lots of bushes where he can hide and rest.
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.
The Pokemon that have been with Ash for a long time, like Snorlax, might have a deeper connection with him and be able to sense that it's still Ash, even though he looks like a hedgehog. Snorlax might be more accepting right away and try to protect Ash from any potential danger. On the other hand, the newer members of his team might be more likely to question whether this hedgehog - like Ash is still their rightful trainer. But as time passes, they would all start to realize that Ash's spirit and his bond with them haven't changed, despite his physical transformation.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
Ash could be a Pokémon trainer with a different starting point in this fanfiction. Born in 1987, by 1997 he might have already made a name for himself in local Pokémon battles. He could have a team of Pokémon that he rescued from difficult situations, like a Charmander that was abandoned by its trainer. His story could revolve around giving these Pokémon a second chance and proving that Pokémon don't need to be 'perfect' to be strong.
The impact of Ash's loss in the fanfiction could extend beyond just himself. His Pokémon might also feel demoralized. However, this could also be an opportunity for them to grow stronger together. Ash could start training more intensively, seek new training methods, or even try to catch new Pokémon to strengthen his team.
One popular type of Pokemon Ash and Pikachu fanfiction might be adventure - based. For example, Ash and Pikachu could embark on a journey to a new and mysterious region filled with undiscovered Pokemon species. They might face challenges like difficult terrains and powerful, previously - unseen Pokemon. In this fanfiction, Pikachu's electric powers could be enhanced in some unique way, and Ash would have to strategize differently to overcome the obstacles.
Well, in certain fanfictions, Ash's inexperience in handling different Pokémon species can make him seem like a bad trainer. He may not be aware of the specific training methods required for some Pokémon. For instance, he might treat a very delicate and rare Pokémon the same way he treats his more common ones, which could lead to the Pokémon not reaching its full potential.
One popular aspect of Alola fanfiction with Ash could be his growth as a trainer. In the Alola region, Ash had unique experiences. Fanfiction might explore how he further developed his skills, formed new bonds with Pokemon, and interacted with the local culture. For example, his relationship with Pikachu might be delved into deeper, showing how they overcame new challenges together in the Alola setting.