There could be a new, evil organization in the fanfiction. Their goal is to use shiny pokemon to power some sort of dark machine or experiment. They target Ash's shiny pokemon because it has a particularly strong energy. This organization would send out their minions to attack Ash and try to capture the shiny pokemon, creating a lot of action - packed scenes in the story.
Obviously, Ash is one of the main characters. He is the trainer who catches the shiny Pokemon. His Pikachu also plays a significant role, as it's always by Ash's side and could potentially have some interesting interactions with the shiny Pokemon.
It could be a jealous trainer. This trainer might see Ash in his shiny pokemon form and want that power for himself. So he tries to capture Ash by any means necessary, setting up all kinds of traps and challenges for Ash and his friends.
It could be Team Rocket. They are always after rare and powerful pokemon. In this fanfiction, they might have a new and devious plan to steal the legendary pokemon that Ash is trying to catch. Their comical yet persistent attempts to thwart Ash would add an interesting dynamic to the story.
A self - proclaimed 'collector' of rare pokemon could be a rival. This person has a huge collection of non - shiny but very rare pokemon and sees Ash's shiny pokemon as the ultimate addition to their collection. They would stop at nothing to get their hands on at least one of Ash's shiny pokemon. And of course, there could be other trainers who simply can't stand the fact that Ash has such an amazing ability and they don't.
One interesting plot could be that Ash catches a shiny Pikachu which has some unique powers, like being able to communicate telepathically with other Pokemon. This would change the way Ash interacts with his team and the challenges he faces in battles.
One possible plot could be that Ash stumbles upon the shiny pokemon by pure chance while exploring a new, uncharted area. Maybe he's looking for a specific type of pokemon for a local competition and instead, he discovers this rare shiny pokemon. Another plot could involve the shiny pokemon being in danger from a group of poachers, and Ash has to rescue it. Then, after rescuing it, he forms a special bond with the shiny pokemon which is different from his bonds with his other pokemon.
It could be a powerful and evil Pokemon trainer who wants to capture Ash in his new Pokemon form. This trainer might be after Ash because he believes that a human - turned - Pokemon has some special power or value.
Maybe a powerful and evil pokemon trainer who has a vendetta against Ash. Now that Ash is in a vulnerable pokemon form, this trainer could try to capture and control him. The trainer might have some dark motives related to using Ash's unique situation to gain power in the pokemon world.
A powerful and greedy Team Rocket member could be the main antagonist. They might be after Diancie because of its rare and valuable powers. They could try to capture Diancie and use it for their own evil purposes, like selling its powers or using it to take over the Pokemon world.
A wealthy and power - hungry trainer who wants to stop Ash from completing his Nuzlocke journey. This trainer believes that the Nuzlocke rules are too restrictive and wants to prove that his way of training Pokemon is superior.