Maybe a group of mean - spirited Pokemon hunters. They see Ash in the dress as an easy target, thinking he's weak or different because of it. They try to capture his Pokemon while he's in this 'vulnerable' state. But Ash, despite the dress, is determined to protect his Pokemon and defeat them.
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.
It could be Malfoy. He might make fun of Harry for wearing a dress and try to use it to humiliate him in front of the whole school, which would then lead to a conflict between them that goes beyond the usual rivalry.
A mysterious figure within Konoha who has a hidden agenda. This person doesn't like the idea of Naruto breaking the norms and wearing a dress. They secretly manipulate events to make Naruto's life difficult while he's in the dress. For example, they might arrange for Naruto to be in situations where his dress - wearing causes maximum embarrassment, all in an attempt to make him conform to the traditional ways.
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.
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.