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.
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.
A jealous male trainer could be the main antagonist. He might see Ash's transformation as an unfair advantage given by some mysterious power. He constantly tries to undermine her in battles and challenges, believing that a female should not be as successful as a male in the Pokemon world.
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.
Maybe a mysterious organization that wants to experiment on Latios. They believe that Latios holds the key to some sort of genetic or power - enhancing technology. They send out their agents, who are skilled in Pokémon capture and combat, to try and get Latios. They don't care about Ash at first, but when he stands in their way, they see him as an obstacle to be removed.
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.
Perhaps a dark - themed Pokémon organization that is researching the unique powers of Mudkip. They see Ash's Mudkip as a key to their experiments. So they send out their agents to capture it. These agents are skilled trainers with powerful Pokémon at their disposal, and they constantly pose a threat to Ash and his Mudkip throughout the fanfiction.
A powerful and greedy Pokémon collector could be the main antagonist. He wants to capture Ash, now in his Mew form, to add to his collection. He has all sorts of high - tech devices and trained Pokémon to try and catch Ash.
Maybe a powerful and envious Mewtwo could be the antagonist. Since Ash has now become a Mew, Mewtwo might feel threatened or think that Ash doesn't deserve such power. So, Mewtwo could try to fight Ash to prove his own superiority.