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.
Perhaps in the fanfiction, Pikachu gains the ability to talk human language fluently. Ash and Pikachu then go on a journey to discover the origin of this new power. Along the way, they face challenges from other Pokémon who are jealous of Pikachu's new ability and from humans who want to use Pikachu for their own gains. This would test their friendship and loyalty to each other.
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.
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.
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.
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.
One possibility could be that Pikachu is somehow mind - controlled. Maybe a powerful and evil Pokémon or a villainous trainer uses a special device or power to make Pikachu turn on Ash. Pikachu might then use its electric attacks like Thunderbolt or Volt Tackle against Ash, leaving Ash shocked and confused as he tries to figure out what's wrong with his beloved partner.
One possible plot could be that Ash's shiny Pikachu is targeted by an evil organization because of its rarity. Ash has to protect it while also trying to figure out why the organization wants it so badly.
Well, perhaps Ash starts to idolize Pikachu after seeing its unique powers in a battle. He might try to imitate Pikachu's moves and even start collecting Pikachu - themed items.