A jealous witch could be the main antagonist. She sees the Sunlight Girl's popularity and power, and out of envy, she tries to cast dark spells on the girl to dim her light.
A shadowy figure from the underworld who is threatened by the Sunlight Girl's ability to bring light and hope. This figure wants to plunge the world into eternal darkness and sees the girl as the main obstacle in his plan.
Since it's named 'the sunlight girl fanfiction', the girl herself is bound to be a main character. There could also be other characters like her friends or family members who support her or are part of her journey. Maybe there's an antagonist as well, someone who is opposed to her because of her sunlight - like qualities, perhaps someone representing darkness or jealousy.
A powerful sorcerer who wants to enslave all the insects for his own dark magic could also be the main villain. Mosqito Girl would have to use her wits and powers to stop him.
It could be an envious rival. Maybe there's another girl who is jealous of Pyro's unique abilities or charm. She constantly tries to undermine Pyro and create obstacles in her path.
Perhaps a character who is allergic to balloons and wants to ban all balloon - related activities in the town. This would put the balloon girl in direct conflict with them as balloons are her passion.
There could be a new type of gem hybrid that is half - corrupted. It has some of the powers of a corrupted gem but also some of the intelligence of a normal gem. This hybrid could be attracted to the unique energy that Mystery Girl and Pearl give off and wants to either absorb it or use it for its own dark purposes.
Perhaps a jealous rival. This person might be a male who is threatened by Ed's success or popularity. When they discover Ed is a girl, they could use this secret to try and drive her out of the group or community she's in. They might spread rumors or create problems for her.
It could be Marik. He might see Yugi's new form as a sign of weakness and try to take advantage of it. Since Marik is often power - hungry, he could target Yugi more aggressively, thinking she'll be an easy target now that she's a girl.
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 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.
There could also be new antagonists. For example, some of the more traditional wizarding families who are against the idea of a female Harry. They might see her as a threat to the old - fashioned ways of the wizarding world and try to undermine her.