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.
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.
In many such fanfictions, Pikachu is often heartbroken when Ash leaves. It might stop eating or become very withdrawn. Some fanfictions show Pikachu trying to find Ash, going on its own journey through the Pokemon world, facing many challenges along the way. It's a really sad concept for the fans who love the bond between Ash and Pikachu.
A really cool fanfiction could be set in a post - apocalyptic world where ash covers everything and Pikachu is one of the few remaining Pokémon. Ash and Pikachu have to survive among the ash leaves, and they discover an ancient power within the Pikachu that is somehow related to the ash. This power could be the key to restoring the world or at least finding a new safe place to live.
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.
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.
First, focus on the setting. Describe the ash leaves in detail - their color, texture, and how they interact with the environment. For example, you could write that the ash leaves crunch under Pikachu's feet as it runs. Then, develop the relationship between Ash and Pikachu. Show how they communicate and rely on each other in this unique setting with the ash leaves.
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.
Brock could be a great addition. He could provide some comic relief and also be the voice of reason. For example, when Ash and Pikachu get into some crazy situations, Brock could be the one to offer practical solutions.
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.