Team Rocket could still be the main antagonists. They might see Ash as a Pikachu and try to capture him more than ever, thinking he's a special Pikachu. Maybe they believe he has some sort of new power because of his transformation.
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.
Maybe the main antagonist is an evil scientist. The scientist has been studying Pokémon transformations and when he sees Ash turn into Pikachu, he wants to experiment on him. He chases Ash around, trying to capture him with all kinds of high - tech devices. This forces Ash to use his Pikachu form's abilities and his human knowledge to outwit the scientist.
One possible plot could be that Ash turns into Pikachu and has to learn how to communicate with his human friends in this new form. He might struggle at first, but then discovers that he can use his Pikachu abilities in unique ways to help the group in battles and in solving problems around them.
One popular type of Pokemon Ash and Pikachu fanfiction might be adventure - based. For example, Ash and Pikachu could embark on a journey to a new and mysterious region filled with undiscovered Pokemon species. They might face challenges like difficult terrains and powerful, previously - unseen Pokemon. In this fanfiction, Pikachu's electric powers could be enhanced in some unique way, and Ash would have to strategize differently to overcome the obstacles.
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.
In this kind of fanfiction, Ash might find it really hard at first. He's used to being a human trainer, and suddenly becoming a Pikachu would be a shock. He'd have to learn how to control Pikachu's electric powers which he only used to command before.
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.
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 Ash, as a female, trying to find her place in the Pokemon league again. Since she has a new form, she might have to re - establish her reputation. She could form new alliances with other trainers who see her in a different light. For example, some male trainers might be more protective or some female trainers might be more competitive towards her. And along the way, she discovers new abilities or a different connection with her Pokemon that was hidden before the transformation.