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.
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.
It could be Team Rocket as always. Since Ash is now a Pikachu, they might see it as an easy target to capture. They could have new gadgets specifically designed to catch Pikachu - Ash, and constantly try to get their hands on him throughout the fanfiction.
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 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.
In this kind of fanfiction, Ash might find it really hard at first. He's used to being a human trainer, and suddenly becoming a Pikachu would be a shock. He'd have to learn how to control Pikachu's electric powers which he only used to command before.
The concept of facing fears is a theme that can be explored. Ash may have fears about not being a good enough trainer, and Pikachu could have fears related to its own power or protecting Ash. Fanfictions can show how they overcome these fears together. Additionally, the theme of legacy could be present. Ash wants to be a great Pokemon master like his idols, and Pikachu is part of that journey. Their story could be seen as part of the greater Pokemon legacy, and fanfictions can play with this idea in different ways.
One possible reason for Pikachu leaving Ash in fanfiction could be a sense of self - discovery. Maybe Pikachu feels it has reached a point where it needs to explore the Pokémon world on its own terms, without Ash. It could be seeking new challenges or to find its true place among other Pokémon.
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.
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.