In the fanfiction, Harry might be on a quest to find a powerful magical item. Along the way, he stumbles upon Pikachu, who is also in search of something, like a rare berry that has special powers. Harry and Pikachu team up. Harry uses his knowledge of magic to help Pikachu navigate through dangerous terrains, and Pikachu, in turn, uses its speed and electric powers to protect Harry from wild Pokemon attacks.
It could be a dark wizard who wants to capture Pikachu for its powerful electric abilities. He might plan to use Pikachu's electricity to power some sort of dark magic device.
One possible plot could be that Pikachu, as a girl, goes on an adventure to find a special type of berry that is said to have magical powers. Along the way, she meets other Pokémon who initially don't accept her because she's different. But through her kindness and determination, she wins them over and they all help her find the berry in the end.
One possible plot could be that Ash realizes Pikachu has grown too powerful for the regular Pokémon battles in their world. So, he releases Pikachu into a special, untouched forest where Pikachu can live freely and hone its skills further without the restrictions of human - Pokémon battles. Pikachu then has to learn to survive on its own and make new Pokémon friends.
A possible plot is that Ash wakes up one day to find that he has swapped bodies with Pikachu. He has to experience life from Pikachu's perspective while still trying to participate in Pokémon battles. This would lead to some really funny and interesting situations as he tries to communicate with the other Pokémon and his human friends in his new form.
Maybe Pikachu gets mind - controlled and turns against Ash, leading to Ash being abandoned. It could also be that Pikachu is forced to leave Ash due to some external danger it doesn't want to expose Ash to.
Perhaps Pikachu discovers a 'dark truth' about Ash. For example, Pikachu might find out that Ash has unknowingly been involved in something that goes against Pokémon ethics, like being associated with a group that mistreats Pokémon. This discovery could make Pikachu feel betrayed by Ash first, and then Pikachu's actions would make Ash feel the same way in return.
Perhaps Pikachu leaves Ash because it has been through a series of really tough battles and has become a bit disillusioned. It might think that by leaving, it can find a new purpose, away from the constant cycle of gym battles and competitions that it has been in with Ash. It may want to explore the world on its own terms, without the pressure that comes with being Ash's main Pokémon.
One possible plot could be that Pikachu is exposed to a rare form of electric energy in a thunderstorm that's charged with some mysterious power. This could start the evolution process. It might struggle at first, not understanding the changes its body is going through. As it evolves, it could gain new abilities like enhanced speed and a more powerful electric attack. And it might have to face new challenges in battles, as opponents are now more cautious of it.
Well, it could start with Ash being transformed into a Pikachu in a mysterious forest. His friends are initially confused but soon realize it's him. As a Pikachu, Ash discovers new abilities he didn't know about before. He might be able to sense things like incoming danger more acutely. He could also have a hard time convincing his other Pokémon that he is still their trainer. Throughout the fanfiction, there could be a series of comical and exciting events as he tries to adapt to his new form and find a way to reverse the transformation.
One possible plot could be that Naruto is on a mission and accidentally stumbles through a mysterious portal and finds himself in the Pokemon world. There he meets Pikachu and they form an unlikely friendship. Naruto, being curious about this new world, tries to learn about Pokemon with Pikachu as his guide.