Who could be the main antagonist in a 'boy turns into pokemon fanfiction'?

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.

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.

It could be a jealous trainer. This trainer might see Ash in his shiny pokemon form and want that power for himself. So he tries to capture Ash by any means necessary, setting up all kinds of traps and challenges for Ash and his friends.

Team Rocket could still be the main antagonists. They might see Ash as a Pikachu and try to capture him more than ever, thinking he's a special Pikachu. Maybe they believe he has some sort of new power because of his transformation.

It could be a group of Team Rocket - like villains. They are after the unique powers that the reborn pokemon might possess. Since the reborn pokemon has a human - like intelligence, they see it as a valuable asset to use for their evil plans, like stealing all the precious pokemon resources or taking over a certain region.

It could be an ancient dark wizard who has been manipulating events from the shadows. This wizard might see Elsword as a powerful tool and has been slowly influencing him to turn dark. The wizard could be using dark magic to cloud Elsword's mind and make him more susceptible to negative emotions.

Bowser could be the main antagonist. He might see Luigi's transformation as an opportunity to mess with Mario and the Mushroom Kingdom. He could try to use Luigi's new form to his advantage, perhaps by trying to make Luigi a spy or by using his new identity to cause chaos among the kingdom's residents.

Perhaps an evil mage who actually intended for the transformation to go wrong on purpose. They might want to use Sans' new female form to cause chaos in the Underground. They could be trying to manipulate Sans into doing bad things or using her to get to other important characters.

It could be a jealous rival. Maybe someone who has always been envious of Robin's abilities or popularity. Now that Robin is in a more vulnerable state as a girl, this rival sees an opportunity to undermine him.

It could be a witch who cast the spell that turned Dean into a girl. This witch might have some ulterior motives, like using Dean's new form to disrupt the balance in the hunting world. Maybe she wants to create chaos among hunters by making them question Dean's abilities now that he's a girl.