The antagonist could be a group of bullies in the story. They pick on Percy because he is now a child and seems weaker. They don't know he is actually Percy with all his heroic qualities. They constantly harass him, steal his things, and try to make his life miserable. This forces Percy and his friends to come up with strategies to deal with these bullies, which could include using Percy's unique skills even in his child form, like his ability to talk to sea creatures to get help when the bullies try to corner them near the water.
One possible plot could be that Percy, as a child, has to navigate his newfound smallness while still having the memories of his older self. He might struggle to convince his friends that he is actually Percy, and they initially think it's some sort of trick. Then, he could use his knowledge from his older self to help them in a new and unexpected way, like solving a mystery that the younger versions of them couldn't figure out on their own.
Morgana could be the main antagonist. She might see Merlin's transformation as an opportunity to further her own plans. Since Merlin is in a more vulnerable state as a girl, Morgana could try to use this to her advantage, perhaps by trying to turn the court against Merlin or using her new form to manipulate her into doing things that would harm Camelot.
It could be an old enemy who now sees Percy as an even greater threat. Maybe Kronos' remnants. Since Percy is now a god, they might fear that he'll be able to fully destroy them. So they try to stop him from getting used to his powers or try to steal his powers.
A power - hungry Viking who sees Hiccup's transformation as a threat to his own authority could be the main antagonist. He might try to capture Hiccup in his Night Fury form and use him for his own evil purposes, like trying to create an army of transformed creatures.
A powerful and corrupt Pokemon trainer could be the antagonist. This trainer is jealous of the boy's unique transformation into a Pokemon. He wants to use the boy's powers for his own gain, like winning all the Pokemon battles and championships. So, he hunts the boy down, causing all sorts of trouble for him during his journey as a Pokemon.
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.
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.
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.
It could be a powerful vampire clan that doesn't want Harry, a new vampire, to disrupt their ways. They might try to either kill him or force him to join their ranks against his will. They could see him as an outsider who doesn't belong in the vampire world and a potential danger to their secrecy.