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.
Well, if Ash turns into a Pikachu in a fanfiction, it would be quite a transformation. Firstly, his communication with Pikachu would become more of an internal dialogue. And he'd have to get used to moving around on all fours and using Pikachu - like movements. His battles would be completely different too. He'd be relying on Pikachu's natural skills rather than his own battle strategies as a human trainer.
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.
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.
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.
Well, perhaps it could be about a magical event that causes Ash to transform into a baby. Then his friends have to take care of him while also trying to find a way to reverse the transformation. Maybe Pikachu has to protect baby Ash from other Pokémon who might mistake him for an easy target.