Well, in this fanfiction, perhaps Zoro's transformation into a girl could be the result of a strange devil fruit power or a magical curse. As a girl, Zoro might face new challenges in the world of pirates. For example, she might be underestimated in battles at first, but then prove her strength in a spectacular way, showing that it doesn't matter what gender the body is, the true strength lies within the soul.
One possible plot could be that Zoro, turned into a girl, has to deal with the different reactions of his crewmates. Some might be shocked, some might tease him. And he has to learn to adapt to his new physical form while still maintaining his tough fighting spirit.
I think Zoro's core personality of being tough and fearless would remain. However, being in a female body might make him a bit more aware of certain things, like how he's treated differently by some people. But overall, he'd still be the same Zoro who doesn't back down from a fight and is always looking to improve his skills.
One possible plot could be that Midoriya is the victim of a quirk gone wrong during a battle or an experiment. As a girl, she might initially struggle with the new identity. She could face challenges in how her friends, like Bakugo and Uraraka, react to her new form. For example, Bakugo might be even more confused and conflicted about his feelings towards Midoriya now. And Midoriya herself has to adapt to the physical and social changes, all while still aiming to be a great hero.
Well, perhaps an enemy uses a magic spell to turn Zoro into a baby as a way to weaken the Straw Hat Pirates. At first, the crew is in chaos trying to deal with a baby Zoro. But as time passes, they see that baby Zoro has a pure sense of determination. He still tries to train, albeit in a very clumsy and cute way. And in the end, they go on an adventure to find the antidote to turn him back, with lots of comical and touching moments along the way.
One possible plot twist could be that Zoro's gender is revealed at a crucial moment, causing shock among the other characters. For example, during a battle when an enemy discovers this secret and uses it to try and throw the team off balance.
Well, one possible plot could be that a villain uses a new type of quirk -altering device on Deku. As a child, he might have to rely on his basic knowledge of heroics and his natural determination. His friends would be confused at first but then quickly realize it's still Deku inside. They'd have to protect him from other villains who might see this as an opportunity to get rid of the future symbol of peace while he's vulnerable.
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.
Well, in a Todoroki turns into a girl fanfiction, perhaps there's a villain who has a transformation quirk and uses it on Todoroki as a form of psychological warfare. Todoroki, now a girl, has to find a way to reverse the effect. In the process, he/she realizes that society treats female heroes differently and has to fight against those prejudices too. Maybe he/she even uses this new form to her advantage in some battles, by using the element of surprise.
One possible plot could be that Cloud, after turning into a girl, has to adapt to a world that now views her differently. Maybe she struggles with newfound emotions and social expectations. For example, she might have been a tough warrior as a male, but as a girl, people underestimate her. She then has to prove herself all over again in battles or quests, while also dealing with her own internal confusion about her new identity.