Well, it depends on the specific fanfic. Some might have a plot where Evil Pikachu teams up with other dark - type Pokémon to take over a region. It could involve battles against the traditional Pokémon heroes.
In some 'pokemon evil pikachu fanfic', the plot might revolve around Pikachu being corrupted by some dark force. This could lead to Pikachu having different abilities and motives than the friendly Pikachu we know. For example, it might start attacking other Pokémon for no good reason and the story could be about how the trainers try to stop it and reverse the corruption.
One possible plot could be Pikachu getting influenced by a dark and powerful Pokemon. For example, a Darkrai might use its nightmare - inducing powers on Pikachu. This could make Pikachu start to attack other Pokemon and trainers without mercy. Maybe Pikachu would first target its own trainer, Ash, as it would be the most shocking turn of events. This would create a lot of chaos in the Pokemon world and force Ash and his other Pokemon friends to try and figure out how to save Pikachu from this evil state.
One idea could be that the female Pikachu has unique electric powers that no other Pikachu has. She could go on a journey to discover the origin of these powers, facing various challenges along the way, like battles with powerful Pokemon and difficult terrains.
Obviously, Evil Pikachu is a main character. There might also be trainers who are trying to either stop it or control it. Maybe some other Pokémon that are allied with or against the Evil Pikachu could also be considered main characters.
One idea could be that Pikachu wakes up in a strange part of the mystery dungeon with no memory of how it got there. It has to team up with other Pokemon it meets along the way, like a Charmander and a Squirtle, to figure out the mystery of the dungeon and also find a way to get back home.
A plot could revolve around Pikachu getting sick and Lillie using her knowledge of Pokémon care to nurse it back to health. During this time, they develop a deep friendship. Then, they notice some strange behavior in other Pokémon around them. Together, they start to investigate and find out that there is a pollution source affecting the Pokémon. Lillie and Pikachu then try to find a way to clean up the pollution and restore the health of the Pokémon in the area. This would show their caring and adventurous sides.
Well, in such a fanfic, a cool plot point might be that Ash, as a Pikachu, has to communicate differently. Since he can't talk like a human anymore, he has to find new ways to convey his thoughts and ideas to his friends. Also, there could be a plot where he discovers new powers that normal Pikachu don't have because he was once human. For example, he might be able to strategize better than a regular Pikachu in battles.
The fanfic could be about Pikachu getting angry when a new powerful Pokemon arrives in the area and starts bullying the other Pokemon. Pikachu, being the brave little Pokemon it is, stands up to the bully. At first, it loses battles due to its anger getting the best of it. But with the support of its friends, it manages to channel its anger into strength and defeats the bully.
One idea could be a story where Pikachu gets separated from the trainer in an unknown and dangerous area, like the mysterious depths of Viridian Forest. It has to find its way back while making new Pokemon friends and facing challenges from wild Pokemon.
The other Pokemon in the Pokemon Center where Pikachu is usually treated would be very frightened. Nurse Joy would be at a loss as to what to do, as she has never seen Pikachu act this way. Professor Oak would be extremely concerned and would start researching immediately to find out what could have caused this sudden change in Pikachu. Team Rocket, on the other hand, might initially see this as an opportunity to capture Pikachu more easily, but they would soon realize that an evil Pikachu is far more dangerous than they expected and might end up getting in over their heads.
One interesting plot could be that the evil teams are texting about a secret plan to steal a rare and powerful Pokemon. They might be discussing how to infiltrate a heavily guarded location where the Pokemon is held, like a special research facility. They could be trading information about security measures and the best time to strike.