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.
Kagome's character development could involve her learning about the responsibilities that come with being a Legendary Pokemon. In the fanfic, she might initially use her powers for her own benefit or to get back to her normal form. But as she meets other Pokemon and trainers who are in need, she starts to understand that she has a duty to protect the Pokemon world. This realization could transform her from a somewhat self - centered girl (in the sense of just wanting to go back to normal) to a selfless hero in the Pokemon world.
Well, it could be special because of its unique take on the Pokémon world. Maybe it has an interesting plot involving Kagome that no one has thought of before.
If Ash becomes a legendary Pokemon in a fanfic, it would change the whole Pokemon world dynamic. He might be hunted by evil organizations who want to use his power. On the other hand, he could also use his new form to protect the Pokemon world better than ever. His friends would likely have a mix of reactions, from being amazed to being worried about losing the old Ash.
Sorry, without reading it, I can't give a proper summary. But it might be about Kagome's adventures in the Pokémon world where she has some sort of legendary connection.
Another possibility is Ash becoming Lugia. Lugia is known as the guardian of the seas. As Lugia, Ash could explore the deep oceans, discover new species of Pokemon, and safeguard the marine ecosystem. He could use his psychic and flying abilities to interact with both water - and flying - type Pokemon, bringing a new era of harmony between different Pokemon types.
His confidence might increase a great deal. Since legendary Pokemon are often seen as very powerful, Ash in his new form would feel more capable. He might become more solitary too, as some legendary Pokemon are known to be lone creatures.
Well, if Ash turns into a legendary Pokemon, he'd likely gain a whole new set of powers. For example, if he becomes Arceus, he'd have the power to create and shape the Pokemon world. He could create new Pokemon species or even reshape the existing regions. His friends would probably be both in awe and a bit scared at first. But Ash, being Ash, would still have his kind heart and would use his powers for good.
In Ash's adventures in legendary Pokemon fanfics, he often has extraordinary encounters. For example, he might form unique bonds with legendary Pokemon that are not part of the main canon. These fanfics can take his story in new and exciting directions, like having him team up with a Mewtwo in a way that defies the normal Pokemon - trainer relationship. It's all about exploring the untapped potential of his character in the context of the amazing world of legendary Pokemon.
In a fanfic, Naruto turning into a Pokemon might also lead to some comical situations. Imagine Naruto, now as a Pokemon, trying to communicate with his friends in Konoha. He might have to use Pokemon sounds and gestures at first, causing a lot of confusion among his comrades. But eventually, they figure out a way to understand him, and it becomes a fun adventure for everyone.