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.
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.
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 really cool plot idea for a Naruto vampire turning fanfic could be that an ancient vampire curse is unleashed in the ninja world. Naruto is the first one to be affected as he's always at the forefront of danger. As he turns into a vampire, he starts to see the world in a different way. His relationships with his friends like Sasuke and Sakura change because he now has different needs and instincts. He might have to learn to control his vampiric side while also using it to his advantage against new, more powerful enemies that are attracted to his new power.
A third plot idea might be that Ash is the chosen one to bring peace between the different factions of legendary Pokemon. There could be some legendary Pokemon that are at odds with each other, and Ash, with his unique bond with Pokemon in general, is the only one who can bridge the gap. He would have to understand the reasons for their conflicts, which could be rooted in ancient history or disputes over territory. Along the way, he'd have to convince both sides that cooperation is better than conflict, and he might use his own experiences with his Pikachu and other Pokemon friends to show them how different species can get along.
In a fanfiction like this, the plot could revolve around the fact that the egg is the key to restoring balance in the Pokemon world. The legendary starter that hatches from it is initially weak but grows rapidly under Ash's care. However, there are forces at work that want to use the starter for their own evil gains. Ash has to team up with his friends and other Pokemon to fend off these threats. The story could also explore the relationship between Ash and the legendary starter as it grows, how they learn from each other, and how their bond becomes unbreakable.
The main character could be the pokemon itself. We could follow its journey as it starts to transform into a legendary. For example, if it's a Squirtle turning into a Kyogre - like form, we can see how it feels about the changes, the new powers it discovers, and how it deals with other pokemon who may be jealous or scared of it.
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.
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.