In a 'luigi turns into a girl' fanfiction, the plot might involve Luigi being transformed due to a strange experiment gone wrong in the Mushroom Kingdom's science lab. His new female form causes a stir among the inhabitants. He could struggle with how others perceive him now. For example, some characters might treat him differently. However, he also uses this opportunity to see the world from a different perspective and becomes more confident in himself over time.
The fanfiction could start with Luigi finding an ancient artifact. When he touches it, he turns into a girl. The story could revolve around his journey to turn back. Along the way, he meets new characters who are intrigued by his unique situation. He might have to learn new skills like dressing up in a more feminine way and dealing with the social norms of being a girl in the Mushroom Kingdom. He could also face some challenges from enemies who see his transformation as a weakness at first, but he proves them wrong as he adapts and overcomes obstacles.
Maybe there's a new villain in the kingdom who has a device that can transform enemies into harmless forms, like turning Luigi into a baby. The villain wants to use this to stop Luigi and Mario from interfering with his evil plans. Mario has to protect baby Luigi from the villain's minions while looking for a way to break the transformation. This could involve exploring new areas of the kingdom and meeting new characters who might have the knowledge to reverse it.
Bowser could be the main antagonist. He might see Luigi's transformation as an opportunity to mess with Mario and the Mushroom Kingdom. He could try to use Luigi's new form to his advantage, perhaps by trying to make Luigi a spy or by using his new identity to cause chaos among the kingdom's residents.
Mario would definitely be a main character. Since Luigi is his brother, he would be the one most concerned about Luigi's transformation and would do everything he could to help him. Princess Peach could also be a main character. She might offer her castle as a safe place for baby Luigi and use her resources to find a solution. Toad could be there too, helping Mario with taking care of Luigi and providing useful information about the kingdom that could lead to reversing the spell.
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.
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.
Well, perhaps the story could start with Keith being exposed to a strange energy. This energy alters his DNA, turning him into a girl. As a girl, he finds that he's now treated differently in society. He uses this experience to understand the struggles that girls face. He might also have to adapt to physical changes like different body strength and new ways of expressing emotions.