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.
Perhaps an organization that believes dragon - type Pokemon are too dangerous and should be controlled. They see Ash as a threat because he is trying to master these powerful creatures. So, they constantly send their agents to stop Ash's progress in his dragon - master journey.
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.
A rogue dragon rider who has different ideas about how to treat dragons. He might see them as tools for power rather than friends like Hiccup does. This rider could be causing trouble for Hiccup and his tribe by stealing dragons or using them to attack other tribes.
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 powerful and evil Pokémon trainer could be the main antagonist. This trainer might have a team of extremely strong and rare Pokémon that he has trained in unethical ways. He could be after some sort of power or rare item that would give him even more control over Pokémon, and Ash stands in his way of getting it.
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.
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.