One possible plot could be that Merlin, after being hit by a strange spell, turns into a girl. His friends in Camelot are initially shocked. Arthur might be both confused and a bit protective in a new way. Merlin has to deal with the new physical form while still trying to protect Arthur and Camelot. She would have to adapt to the different expectations placed on women in Camelot, like different dress codes and social behaviors, all while hiding her true identity as the powerful sorcerer Merlin.
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.
One possible scenario could be that in this fanfiction, the change forces Kratos to view the world from a different perspective. As a girl, he might face different challenges and reactions from the other characters. Maybe the gods who once feared him now underestimate him, leading to some interesting power dynamics.
Perhaps in a parallel universe, Superman is born as a girl. This version of her grows up with different experiences and challenges. She could be more in tune with female issues in society and use her powers to fight for gender equality as well as the usual superhero stuff like saving the world from villains and disasters.
Another possibility is that Finn - now a girl - discovers new powers or abilities that were hidden before. Since in many stories, a change in gender can sometimes bring out different aspects of a character. She could use these newfound powers to fight against the villains in Ooo in a more creative way than before.
Maybe in this fanfiction, Midoriya as a girl discovers new aspects of her power. Since her body has changed, the way she channels her quirk might also be different. She could then have to train all over again to master this new way of using her ability.
Physically, of course, there would be obvious changes. But personality - wise, he might become more in touch with his emotions. In the original, Lelouch was often very calculating and hid his true feelings. As a girl, society in the story might expect her to be more open about her emotions, which could force her to confront and express them more.
One idea could be that Steven turns into a girl due to a magical mishap. Maybe he was trying to help a gem with a spell and it backfired. As a girl, he could struggle with how others perceive him differently, but also discover new aspects of his personality. He might have to learn new ways to fight and use his powers, adding an interesting twist to the story.
A possible fanfiction plot could involve Sherlock's friends' reactions to his transformation. Watson might be initially shocked but then adapt and see that Sherlock is still the same brilliant mind, just in a different form. Mycroft could be more concerned about how this transformation affects Sherlock's standing in the intelligence community. And Lestrade might be confused but still rely on Sherlock's skills to solve difficult cases, albeit with some awkward moments at first due to the change.
One possible scenario in this fanfiction could be that Naruto uses some sort of jutsu accidentally. Maybe he was trying out a new technique and it misfired, turning Kiba into a girl. In fanfiction, this could lead to a lot of humorous situations. For example, Kiba, now as a girl, might struggle with how to act or how to deal with the reactions of his friends like Shino and Akamaru. Also, Naruto would likely feel really guilty and try to find a way to reverse the transformation as soon as possible.