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.
Team Rocket could still be the main antagonists. They might see Ash as Pikachu and think it's a new, rare Pikachu. So they try to capture him more desperately than ever. They could have some new gadgets designed specifically to catch this 'strange' Pikachu which is actually Ash.
One possible plot could be that Ash is hit by a strange new type of Poké - ball created by a mad scientist. This ball has the power to transform trainers into Pokémon. Ash wakes up to find himself in the body of Pikachu. At first, he's disoriented but then realizes he can communicate with his other Pokémon in a new way. He teams up with Pikachu (who is now acting a bit like Ash) and they go on an adventure to find the scientist and reverse the transformation.
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.
A new, powerful and evil Pokemon trainer could be the antagonist. This trainer might be jealous of Ash's new form and powers as a Pokemon and wants to capture him to use his powers for his own gain. He could set up various traps and battles to try and subdue Ash.
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.
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.
One possible plot could be that Ash is transformed into a rare and powerful Pokemon like Mewtwo. He then has to navigate the Pokemon world from a different perspective. He discovers new abilities and strengths he didn't know about before. And he uses his knowledge of being a trainer to communicate with other Pokemon and form alliances to face the challenges that come his way.