It could be a powerful and jealous trainer who wants to take Ash's place. This trainer might use dark magic or some underhanded tactics to make Ash's Pokemon abandon him so that he can swoop in and claim them for his own.
Perhaps an evil organization that has a vendetta against Ash. They might target his relationship with his Pokemon by spreading lies or using some kind of mind - control device on the Pokemon to make them leave Ash. Their goal could be to stop Ash from achieving his dreams and disrupting their own evil plans in the Pokemon world.
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.
A new, powerful and evil Pokemon trainer could be the antagonist. This trainer might be jealous of Ash's new form and powers as a Pokemon and wants to capture him to use his powers for his own gain. He could set up various traps and battles to try and subdue Ash.
In this kind of fanfiction, perhaps Ash loses a big Pokemon battle and his Pokemon feel disappointed and abandon him. This could lead to Ash going on a journey of self - discovery to regain their trust and friendship.
It could be a jealous rival trainer. This trainer might have always been envious of Ash's popularity and the loyalty of his harem. So, they plot and scheme to turn the harem against Ash by spreading lies and creating discord among them.
It could be a rival trainer who is jealous of Ash's success and also wants to use Princess Sara for her influence. He spreads false rumors about Ash to turn the people of the princess's kingdom against him. He then tries to take over the role of the hero in the situation, but his real intentions are to gain power and wealth by any means necessary.
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.
Maybe a new villain organization that believes in a different way of training Pokemon, a cruel and unethical way. They see Ash as a threat to their ideology because he is a symbol of the traditional, kind - hearted Pokemon training. So they constantly try to stop him from achieving his goal of becoming a Pokemon master. For example, they might try to steal his Pokemon or sabotage his battles.