Another aspect could be that Bill's transformation of Dipper into a girl has some unforeseen consequences. It could be that there is a magical imbalance in the Gravity Falls world as a result. Dipper, in his new form, might discover new powers or abilities that he has to learn to control, all while trying to find a way to reverse the transformation and stop Bill's machinations.
One possible plot could be Dipper's struggle with his new identity. He might have to deal with how his friends and family react. For example, Mabel could be super excited to have a sister for a change. And Dipper would have to navigate a world that treats him differently as a girl, like different expectations in Gravity Falls' strange society.
Obviously, Bill could be the main antagonist. Since he's the one who turns Dipper into a girl, he's likely up to no good. His motives are usually self - serving and evil, so he would probably be causing more trouble for Dipper and the other characters in Gravity Falls.
Well, perhaps it's a story where Dipper gets caught up in Bill's gang's evil plans. They might try to manipulate him into doing their bidding, but Dipper uses his wits to outsmart them. The plot could involve Dipper having to navigate through a series of dangerous situations created by Bill's gang, all while trying to protect his friends and the town he loves.
Well, it could be about Bill having some strange and powerful reasons to choose Dipper as his 'queen'. Maybe he sees Dipper's unique qualities like intelligence or a special connection to the supernatural world in the Gravity Falls setting. He might then use his otherworldly powers to transform Dipper into a sort of queen - perhaps giving Dipper powers or a new status that comes with responsibilities and challenges in this fan - created world.
One possible plot could be that Bill Cypher, in human form, befriends Dipper while hiding his true chaotic nature. As they get closer, Dipper starts to notice strange things about his new friend. Eventually, Bill reveals his true self but by then Dipper has grown to understand him a bit and tries to find a way to either change Bill or at least keep his chaos in check.
If Dipper turns into a girl in the Gravity Falls fanfiction plot, it would likely cause a stir among the other characters. Mabel would probably be excited at first but then have to adjust to this new version of her brother. The plot could then focus on Dipper trying to find a way to turn back. Along the way, he might discover new things about the town or himself. For instance, there could be a new magical entity that only reveals itself to Dipper in his female form, leading to a whole new adventure.
A possible plot could focus on how Tails' daily life changes. As a girl, she might have to deal with different social expectations. Her hobbies and interests could shift slightly. For example, she might be more interested in certain types of fashion or more nurturing activities. Her relationship with Amy could also change, perhaps they become closer friends as they share more 'girly' things. And Tails' mechanical skills might be seen in a new light, as she uses them in a different way to solve problems in her new form.
Well, one possible plot could be that Sans somehow gets hit by a strange magic spell. As a girl, he/she might struggle to fit into the existing relationships in the Underground. For example, Papyrus might be really confused at first but then tries to be supportive. And Sans (as a girl) could also have different interactions with Frisk, like maybe a more motherly or sisterly bond.
Well, perhaps in the fanfiction, Ash is exposed to a mysterious Pokémon's special ability that causes the transformation. As a girl, Ash discovers new aspects of himself. He might find that some Pokémon are more attracted to his new form and are more willing to be caught or trained. He could also use this new identity to his advantage in battles, like using the element of surprise against opponents who are used to facing the male Ash.
In a fanfiction where Nagisa turns into a girl, it could be part of a wish - fulfillment scenario. Nagisa secretly wished to experience life as a girl for a day, and some supernatural being grants that wish. As a girl, Nagisa has to navigate through daily life, school, and social situations. There could be humorous moments when Nagisa tries to act like a girl but still has some of the old habits from being a boy, like being too rough during sports in a girl's body.