Well, it could be about Ben 10 being transformed into a girl and then having to deal with the differences in how society treats him/her. Maybe he/she has to face new challenges in using the Omnitrix while being a girl.
One possible plot for this fanfic could be that Ben 10, now as a girl, has to enter a world that is more gender - specific in terms of the challenges it presents. For example, in certain alien cultures that he/she encounters through the Omnitrix, being a girl might have different implications. He/she may have to fight against gender stereotypes within those cultures while still using his/her powers to save the day. Also, the way he/she interacts with female characters in the Ben 10 universe could change drastically, leading to new storylines and character developments.
Well, maybe Ben 10 gets transformed into a girl by some alien device malfunction. Then he has to deal with all the new experiences like how people treat him differently, and he might discover new powers related to his new form.
Another possibility is that an alien race that Ben has previously encountered and thought was friendly, now turns against him/her because of the change in gender. They might have some strange beliefs or customs that make them view Ben's new form as a threat or an abomination, and thus they become the main antagonists in the fanfic.
In a 'sonic becomes a girl fanfic', the story could start with some kind of magical or scientific accident that turns Sonic into a girl. She then has to navigate her world with a new body. She may discover new abilities or limitations. Perhaps she teams up with female characters in the Sonic universe more often now and they form a strong girl - power group to take on the bad guys. There could also be a focus on how Sonic adjusts to her new appearance and how it affects her relationships with characters like Knuckles, who may have a different dynamic with her now.
In a 'sans becomes a girl fanfic', perhaps Sans - now a girl - decides to use this new form to her advantage. She could try to infiltrate a group that was previously only male - dominated in the Underground. This would lead to a whole series of adventures as she tries to keep her true identity a secret while achieving her goals.
Well, it could be about Ash waking up one day as a girl and having to deal with all the changes in his, now her, life. Maybe she has to adjust to new social dynamics, different reactions from friends and rivals, and also how to handle Pokémon battles in a different body.
Well, it could be about Gohan's daily life experiences as a girl. Maybe he has to deal with different social situations, like how his friends and family react to his new form.
In the 'izuku becomes a girl fanfic', perhaps the plot involves Izuku adjusting to her new physical form. She might struggle with things like different clothing styles and how society views girls in the hero world. Also, her relationship with other characters could change. Her friendship with Ochaco might become more complex as they now have more in common as girls. And Bakugo might react in a really interesting way, either being more protective or even more competitive towards her.
Well, perhaps the story could be about Harry suddenly waking up in a girl's body. He might struggle with the new physical appearance and how people around him react. For example, his friends might be really confused at first.
In this 'natsu becomes a girl fanfic', it could center around the idea that Natsu as a girl discovers a whole new side of the magical world. Since he is now a girl, he might be able to access certain areas or knowledge that were previously off - limits. He could also form new relationships with other characters who see him in a different light.
One possible plot could be that Ryuuji wakes up one day as a girl. He, now she, has to deal with the reactions of her friends like Taiga. Taiga might initially be shocked but then grow more protective. Ryuuji would also have to adjust to new social dynamics at school, like how the boys interact with her now and how she fits in with the girls. She might struggle with wearing different clothes and having different expectations placed on her.