Well, one possible plot could be that Mario touches a magical item in the Mushroom Kingdom. This item has the power to transform him. As he turns into a girl, he discovers new abilities. Maybe he can now access areas only females can in the kingdom. He also has to deal with how his friends like Luigi react to his new form.
Luigi would probably be very surprised at first. He might be a bit confused, but then he would support his 'sister' Mario. Princess Peach would be delighted. She could finally have a female friend who has all of Mario's skills. Bowser might not even notice at first, thinking it's just another ploy by Mario. But when he realizes it's a real transformation, he might be a bit more hesitant in his attacks, not sure how to deal with this new female Mario.
A plot could involve the shy girl being a huge admirer of Mario from afar. She is too shy to talk to him directly but leaves little anonymous gifts for him. Mario eventually catches her in the act. At first, she's embarrassed but Mario is kind and encourages her to be more open. They end up going on an adventure together where the shy girl discovers her own hidden strengths.
Well, one possible plot could be that Deku accidentally drinks a strange potion created by a villain. This potion has the side - effect of changing his gender to female. At first, he's shocked and confused. But then, he realizes that he can use this new form to his advantage in hero work. For example, he might be able to infiltrate a female - only villain group more easily. He also has to deal with his friends' reactions, some are surprised while others are supportive and help him adjust to his new body while still being a hero.
Well, the plot could be that Link is cursed to turn into a girl. In this new form, she discovers new powers or abilities that were hidden before. Maybe she can communicate with certain creatures better as a girl. And she embarks on a journey to find a way to reverse the curse, all while facing challenges not only from the monsters but also from the people who don't trust her in this new form.
Well, perhaps in the fanfic, Ash turns into a girl because of a curse. As a girl, Ash could start to see the Pokemon world from a different perspective. Maybe she would form new relationships with her friends, like Pikachu being more protective. And she might have to enter Pokemon contests instead of battles at first due to the new appearance, but then find a way to get back into battles and show that she's still the same strong trainer inside.
In a 'kuroko turns into a girl fanfic', perhaps there's a magical event that causes the transformation. Kuroko, as a girl, could discover new aspects of his/her personality. For example, he/she might find that he/she is more in touch with emotions or has a different sense of style. And there could be a whole story about how he/she tries to fit into this new body while still maintaining his/her old relationships and goals.
One possible plot could be that Levi is subjected to a magical spell or some strange scientific experiment gone wrong. As a girl, he (now she) has to navigate the world in a different way. Maybe she struggles with the new social expectations and body, but also discovers new aspects of herself and her relationships. For example, she might find that her friends interact with her differently, and she has to re - evaluate her place within the group.
A 'sonic turns into a girl' fanfic could have a plot where an evil villain casts a spell on Sonic to turn him into a girl as part of a devious plan. Sonic, now in a female form, has to find a way to break the spell. Along the way, Sonic realizes that being a girl gives her new insights into the challenges that female characters in her world face. For example, she might notice how some areas are less accessible or how she is treated differently by some characters, which could lead to a story about equality as well as Sonic's quest to return to his normal form.
Well, it could start with Izuku waking up as a girl one morning with no explanation. He (now she) tries to keep it a secret at first but then realizes it's not possible. The story could focus on how Izuku's relationships change. For example, Bakugo might initially tease her more harshly but then come to respect her in a different way. And Midoriya could find that she has a different approach to using her quirk as a girl, maybe being more strategic or having different physical capabilities that she has to learn to master.
Well, in a 'freed turns into a girl' fanfic, the plot might start with an experiment gone wrong. Freed was perhaps trying to enhance his magic power and something in the experiment caused his physical form to change to that of a girl. He would have to deal with the reactions of his guildmates. Some might tease him, while others would be more understanding. And he would also face challenges like how to adapt to the new body's physical limitations while still being able to participate in guild activities.