There could be a powerful and jealous trainer who has always been in Ash's shadow. This trainer decides to use underhanded tactics to prevent Ash from becoming a Pokemon master. He might set traps for Ash during his journey, or try to spread false rumors about Ash to turn other trainers against him.
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.
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.
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.
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 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 in using Pokemon for cruel and unethical experiments. They view Pokemon as mere tools and are trying to create super - powerful Pokemon through these experiments. They see the main character, who has a more harmonious relationship with Pokemon, as a threat to their plans.
It could be a former ally of the Pokemon Master who has turned evil. Maybe they felt overshadowed by the Pokemon Master and now seek revenge. For example, they might have found a dark artifact that corrupts them and gives them the power to control other trainers' Pokemon.
Team Rocket from the Pokemon world could be the main antagonist. They might see Naruto as a threat to their plans of stealing powerful Pokemon. They could try to use their usual sneaky tactics on Naruto, but he would be too smart for them with his ninja training.
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.