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.
It could be that Pikachu meets a Pokémon trainer who has a special connection with electric - type Pokémon in a way that Ash doesn't. This new trainer might offer Pikachu a different kind of training or lifestyle that seems more appealing at the moment. So, Pikachu decides to leave Ash to join this new trainer, hoping for new experiences and growth.
Well, Ash would be devastated. Pikachu has been his closest companion throughout his Pokémon journey. He might go on a desperate search for Pikachu, traveling to all the places they've been together. This could lead to Ash encountering new Pokémon and trainers along the way.
An ancient and powerful Pokémon with the ability to influence Pikachu's mind could be the antagonist. It might see Pikachu's potential and wants to use it for its own dark purposes. By making Pikachu leave Ash, it can start to manipulate Pikachu more easily.
In some fanfictions, Pikachu could inherit Ash's dream. Ash had a big dream of becoming a Pokemon Master, and Pikachu might take up that mantle in a way. It could start traveling to different regions, challenging strong trainers, all while carrying the memory of Ash with it. This would show how much Ash influenced Pikachu during their time together.
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.
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.
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.
The plot might involve a lemon - shaped Pokémon that is being hunted by an evil organization. Ash and Pikachu vow to protect this unique Pokémon. They have to outwit the organization's traps and battles. For example, Pikachu could use its Thunderbolt to break through electric fences set up by the organization, while Ash uses his strategic skills to plan their escape routes. This would lead to a series of exciting adventures and battles as they strive to keep the lemon - shaped Pokémon safe.