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.
The plot could involve the shiny Pikachu having a unique power that only Ash can unlock. As Ash tries to understand and control this power, he goes on a journey to find a Pokémon sage who can teach him more about it. Along the way, he has to battle other trainers who are jealous of his shiny Pikachu.
There are. A lot of fanfiction writers like to come up with unique scenarios involving Ash and a shiny Pikachu. Some stories might have the shiny Pikachu possess special abilities that normal Pikachu don't have. It could make Ash's journey even more exciting as he has to deal with the attention that a shiny Pokémon attracts from other trainers and Pokémon alike.
The plot could involve Ash starting his journey in a unique way. Since he has a shiny Charmander, it might attract the attention of Team Rocket more than usual. They could constantly try to steal it, creating many action - packed chase scenes. Ash would have to train his Charmander to be stronger to fend off these threats. Along the way, he could meet other trainers with rare Pokémon who are both friends and rivals, all while aiming for Pokémon League championships.
In this fanfiction, Ash could start in a mysterious forest and spot a shiny Celebi. Celebi, being a time - traveling Pokemon, could take Ash on a journey through different time periods in the Pokemon world. He might see how the Pokemon world has evolved over time, meet some of his ancestors or future versions of himself, and have to solve various problems in each time period to keep the balance of the Pokemon world.
Another possibility is that the shiny Riolu has some special powers that Ash discovers. They could be related to a hidden legend in the Pokemon world. Ash and the Riolu then have to protect this secret from those who would use it for evil. They might travel to ancient ruins and encounter other mysterious Pokemon on this journey.
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.
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.