Well, perhaps Ash starts to idolize Pikachu after seeing its unique powers in a battle. He might try to imitate Pikachu's moves and even start collecting Pikachu - themed items.
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.
His relationship with Pikachu would become quite different. Pikachu might have to take on a more protective role. Instead of Ash being the one in charge, Pikachu would be looking out for Eevee - Ash. They would still have a strong bond, but it would be based more on Pikachu's guardianship now.
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.
In the 'Ash becomes a Pikachu fanfiction', Ash could have dreams where he is actually a Pikachu. This could lead to some really interesting adventures. For example, he wakes up in a dream as a Pikachu and has to figure out how to use his new form to battle. He might also make new Pikachu friends in his dream world and learn their unique ways of living. This could then have an impact on how he treats Pikachu in the real world when he wakes up, like being even more protective and affectionate towards his beloved Pikachu.
Ash would be completely devastated. Pikachu has been his closest companion since the start of his journey. He would probably go into a state of shock at first, unable to believe that Pikachu was gone.