One idea could be that a regular pokemon gets exposed to a strange, ancient energy that starts its transformation into a legendary. For example, a Pikachu accidentally stumbles upon an old, forgotten shrine in a forest and absorbs some of the mysterious energy there. Another plot could involve a pokemon trainer who has a special bond with their pokemon, and through that bond, the pokemon begins to evolve into a legendary form to save the trainer from a great danger. Maybe a Charizard turns into a Moltres - like form to battle an extremely powerful enemy threatening its trainer's life.
A plot idea could be that a group of scientists are conducting experiments on pokemon genetics. They accidentally create a situation where a pokemon starts evolving into a legendary. This could lead to chaos as the pokemon's new powers are not fully understood. Also, in a more magical setting, a pokemon might be chosen by a magical entity to turn into a legendary to fulfill a prophecy. For instance, a Eevee could be chosen to become a Suicune - like pokemon to bring balance to a region suffering from droughts and floods.
Perhaps a pokemon is transported to an alternate dimension where the rules of evolution are different. There, it starts to turn into a legendary. This could be due to the unique energy or environment of that dimension. As it transforms, it has to learn to control its new powers while also finding a way back to its own world. The pokemon could be a Bulbasaur, and as it turns into a Venusaur - like legendary, it discovers new plant - based powers that it didn't have before.
One idea could be a person turning into a Pikachu and having to navigate the Pokemon world while still retaining some human memories and emotions. They might struggle with using Pikachu's electric powers at first.
A plot could involve a human accidentally stumbling into the hidden realm of Legendary Pokemon. There, they have to prove themselves worthy to the Pokemon and in the process, they develop a unique friendship. The human then uses this new - found relationship to bring peace between humans and Legendary Pokemon who have long been in conflict due to misunderstandings.
One idea could be Ash encountering a Legendary Pokemon that has lost its memory. Ash then tries to help it regain its memories while protecting it from poachers. Another could be a time - travel plot where Ash is sent back in time and has to team up with a Legendary Pokemon to prevent a great disaster from happening in the past that would change the future of the Pokemon world.
A great plot could involve a world where Skin Pokemon are the outcasts because their changing abilities are seen as untrustworthy. A group of trainers decides to prove that Skin Pokemon are just as loyal and worthy as any other type. They form a sort of rescue team for Skin Pokemon in danger and work towards changing the perception of these unique creatures in the Pokemon world.
Well, a plot could revolve around Janine being involved in a Pokemon tournament where she uses her unique Poison - type Pokemon in unexpected ways. She might face some opponents who underestimate her because of her small stature but she proves them wrong with her brilliant strategies. Or perhaps a story where Janine discovers an ancient Pokemon artifact that has the power to control all Poison - type Pokemon and she has to protect it from evil forces.
One idea could be that Kagome's transformation into a Legendary Pokemon is a result of a strange encounter with a powerful Pokemon artifact in the Pokemon world. Maybe she was exploring an ancient ruin and accidentally activated a spell that merged her with the essence of a Legendary Pokemon, like Mew. This could lead to her having unique powers and abilities that no other trainer has.
One idea could be a group of friends on a scientific expedition who accidentally get exposed to a strange energy that starts to turn them into Pokemon. Another could be a single character who wishes so hard to be a Pokemon to escape a difficult human life and slowly begins the transformation. Maybe a whole town gets affected by a magical spell that turns them into Pokemon and they have to figure out how to reverse it while living as Pokemon.
One idea could be a baby Pokemon getting lost while wearing diapers and the other Pokemon in the area have to help it find its way back home. Another could be a trainer who discovers a rare Pokemon in diapers and has to figure out how to take care of it while also protecting it from those who might want to steal it for its rarity.
One idea could be a trainer in the Pokemon Sumo world who discovers a rare Pokemon with unique sumo - like abilities. Another could be a Pokemon sumo tournament where the underdog Pokemon rises to victory. Or perhaps a story about a trainer who has to train their Pokemon specifically for sumo battles in a new region.
One idea could be exploring the backstory of how Professor Oak started his research on Pokemon. Maybe he had a special encounter as a young trainer that inspired him to study Pokemon evolution and behavior.