Perhaps an ancient Pokémon spirit that sees Ash's new powers as a threat to the natural order of the Pokémon world. It tries to stop Ash from using his aura psychic abilities by creating various obstacles and sending other Pokémon to battle him.
A new villain could be introduced. For example, a powerful trainer who is jealous of Ash's new powers. This trainer wants to defeat Ash and steal his aura psychic abilities to become the most powerful trainer in the world.
Maybe there's a power - hungry rival from Ash's own group who becomes jealous of his newfound ability and turns against him. This rival could then become the main antagonist, constantly trying to undermine Ash and take his power.
Maybe an ancient evil spirit that has a vendetta against Ash. The spirit could possess the aura guardian and make it turn against Ash. In this scenario, the main antagonist is not really the guardian but the spirit that controls it, and the story would be about Ash fighting this spiritual evil.
It could be that there is a young, impressionable character who looked up to Ash Aura. After the betrayal, this character is confused and starts to question his own beliefs. There might also be a powerful mage in the story who has the ability to see the truth behind Ash Aura's actions and is trying to prevent the chaos that the betrayal has caused.
It could be a jealous rival trainer who mocks Ash for wearing a dress and tries to use it as a way to undermine his confidence and stop him from competing in Pokemon battles.
One interesting plot could be Ash discovers he has an aura - related psychic power that allows him to communicate with Pokémon on a deeper level. He uses this power to understand their emotions and needs better, leading to stronger bonds and more successful battles.
It varies depending on the specific fanfiction. Some might give Ash these powers early on, while others might introduce them later in the story.
A powerful and evil Team Rocket member could be the main antagonist. This person might be after Lucario because of its rare abilities. They could try to capture Lucario to use it for their own evil plans, like taking over the Pokémon world or stealing valuable Pokémon. Ash would then have to fight against this Team Rocket member to protect Lucario.
A jealous rival who also wants to be known as the dragon master. This person would try to undermine Ash at every turn, spreading false rumors about him and trying to steal his dragon - taming techniques.
Maybe Team Rocket could be the main antagonist. They might see Ash's transformation as an opportunity to finally capture Pikachu. Since Ash is in a more vulnerable state as a girl, they could try to use different tactics to take advantage of the situation.
Perhaps a jealous trainer. This trainer has always been envious of Ash's success and now that Ash is in a vulnerable state as a Zorua, the trainer sees an opportunity to get rid of him for good. The trainer could try to use his own Pokémon to capture Ash or even harm him, making the trainer a formidable antagonist in the story.