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.
It could be a misunderstanding. There might be a Pokémon that can shapeshift into Pikachu. This fake Pikachu could perform actions that make it seem like the real Pikachu has betrayed Ash. Ash would be heartbroken, and the real Pikachu would have to prove its innocence and fight to regain Ash's trust once the truth is uncovered.
In a 'ash and pikachu snivy betrayed' fanfiction, the betrayal could stem from a misunderstanding. Maybe Snivy overhears Ash making a plan that it misinterprets as something harmful to the Pokémon world. As a result, Snivy feels it has no choice but to betray Ash and Pikachu to protect what it believes in. It could then side with a group of anti - human Pokémon who are also misinformed about Ash's intentions.
It could be a powerful and jealous Pokémon trainer. This trainer might want to get rid of Ash so he can be the best. He manages to brainwash Snivy into betraying Ash and Pikachu. This trainer would be the main antagonist, constantly trying to stop Ash from achieving his goals and causing trouble for him through Snivy's betrayal.
Ash would be extremely devastated. Pikachu is not just his Pokémon but also his closest friend. He would likely first try to reason with Pikachu, not believing that Pikachu could truly betray him. He might be in denial for a while.
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 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.
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.
One possible plot could be that Ash starts his journey with a different starter Pokémon, like Bulbasaur. Without Pikachu by his side, he might face different challenges and form different relationships. He could struggle more in the early battles without Pikachu's electric power, but then develop unique strategies with his new Pokémon team.
Well, it could be that Ash makes a discovery that goes against Arceus' grand plan and is thus betrayed. Maybe he finds out something about the balance of the Pokemon world that Arceus doesn't want known.