Who could be the main antagonist in 'Aang turns into a girl fanfic'?

It could be an evil wizard who was the one who actually caused Gumball to turn into a girl in the first place. The wizard has some ulterior motives, like using Gumball's new form to gain power or to cause chaos in the town. He tries to prevent Gumball from finding a way to turn back and keeps creating obstacles for Gumball and her friends.

It could be a villain who is obsessed with creating chaos through strange transformations. This villain might have targeted Deku specifically because of his status as a rising hero. They want to see how Deku will cope with being turned into a girl and use it to undermine his confidence and hero image.

Ganon could still be the main antagonist. Even though Link has turned into a girl, Ganon's goal of taking over Hyrule remains the same. He might underestimate Link in her new form, which could give Link an advantage. For example, Ganon might not expect the girl - Link to be as strong or resourceful as the male Link, but she proves him wrong.

Perhaps a jealous classmate who has always been in Izuku's shadow. Now that Izuku is a girl, this classmate sees an opportunity to undermine her. They spread rumors about how Izuku's transformation is some sort of cheat or that she's not the same reliable person she was as a boy. This classmate's actions and attempts to sabotage Izuku would make them the main antagonist.

It could be a jealous trainer who doesn't like the attention that girl - Ash is getting. This trainer might try to undermine her at every turn during Pokemon battles.

Maybe a new character could be the main antagonist. This character could be jealous of Sonic's speed and popularity. So, when Sonic turns into a girl, this antagonist sees it as a chance to frame Sonic for various crimes in this new form. This would make Sonic's friends doubt him and isolate him, all part of the antagonist's plan to take Sonic's place as the hero of their world.

Perhaps a new alchemist who is jealous of Edward's skills, even in his new form. This antagonist might see Edward as a girl as an easy target to discredit. They could spread rumors about how Edward's transformation has made him less capable in alchemy, when in fact it's just their way of trying to undermine him.

Maybe it's a group of purist wizards who form a secret society. They are determined to 'purify' the wizarding world by getting rid of anyone or anything that is not a normal human wizard. Harry, as a monster girl, becomes their prime target. They use dark magic and underhanded tactics to try and capture or kill Harry.

Well, one possible plot could be that Aang is hit by a strange magic or spell that turns him into a girl. He then has to deal with the new physical form while still trying to fulfill his duties as the Avatar. Maybe he discovers new aspects of the world around him that he didn't notice before as a boy, like different social expectations for girls in the Avatar universe.

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.