One possible plot could be that a boy, who is a huge Pokemon fan, somehow gets zapped by a strange energy during a Pokemon event. He then slowly starts to transform into his favorite Pokemon, say Pikachu. As he turns into Pikachu, he discovers he can still think like a human but has all the powers of Pikachu. He then has to figure out how to reverse the transformation while also using his newfound powers to help other Pokemon in danger.
In this fanfiction, the boy could be a big Umbreon fan. One day, he finds a strange amulet while on an adventure. As soon as he touches it, he begins to change into Umbreon. He then discovers that he has new abilities like being able to move silently in the dark and use moves like Dark Pulse. He decides to use his new form to protect his town from a group of wild Pokemon that have been causing trouble.
In this fanfiction, if Ash turns into a pokemon, perhaps he becomes a Charmander. He could struggle with being in a different form. He might have to adapt to the life of a fire - type pokemon, facing challenges like being weak against water - type attacks and trying to find a way to change back while still having adventures with his friends, who now have to protect him in a different way.
One possible plot could be that Ash gets hit by a strange Pokemon move during a battle. This move has a side - effect of changing his gender. As a female, Ash discovers new perspectives within the Pokemon world. She might form different relationships with her Pokemon, especially the female ones who now see her as one of their own. She could also face challenges from other trainers who underestimate her because she's now a girl, but she uses her newfound determination and different approach to battles to prove them wrong.
The plot might involve Sonic being turned into a Pokemon by a powerful wizard in the Pokemon universe. As a Pokemon, he retains some of his speed abilities but also has to learn Pokemon moves. He meets a group of friendly Pokemon trainers who help him understand this new world. Together, they enter Pokemon battles against other trainers and gym leaders, all while Sonic tries to find a way back to his own world.
Maybe Serena discovers the exciting world of Pokemon and gets completely hooked. She could start by seeing a Pokemon battle and being in awe of the power and beauty of the Pokemon. Then she might decide to start her own journey to catch different Pokemon, facing various challenges along the way.
A powerful and corrupt Pokemon trainer could be the antagonist. This trainer is jealous of the boy's unique transformation into a Pokemon. He wants to use the boy's powers for his own gain, like winning all the Pokemon battles and championships. So, he hunts the boy down, causing all sorts of trouble for him during his journey as a Pokemon.
Well, it could be that Ash's transformation into a Pokémon gives him unique abilities. Maybe he can communicate with Pokémon in a new way and goes on an adventure to discover the reason for his transformation. However, 'lemon' in fanfiction often implies inappropriate content, so if we assume a non - inappropriate version, his new form might also lead to him having to prove himself to his friends all over again.
One possible plot could be that Danny Phantom, while exploring some sort of interdimensional rift, gets zapped by a strange energy that transforms him into a Pokemon. Maybe he becomes a Pikachu. He then has to figure out how to use his new Pokemon abilities to get back to his normal form. He might start by trying to communicate with other Pokemon, learning their ways and using his ghostly knowledge to his advantage in the Pokemon world.
It could start with the boy having a really strong wish to be a Pikachu. His intense desire somehow triggers a magical event that turns him into one. Once he's a Pikachu, he discovers that the Pokémon world is not as simple as he thought. He has to face challenges like avoiding being caught by Pokémon trainers and finding food suitable for a Pikachu in the wild.
Well, it could be about a scientific experiment gone wrong. A boy volunteers for an experiment that is supposed to enhance human abilities. But instead, there is a malfunction in the machinery or chemicals used. This causes his genetic makeup to change, and he turns into a girl. He then has to deal with his new identity, his family's reaction, and his own feelings about this sudden transformation.