In a Belle turns into a beast fanfic, the plot might involve a magic experiment gone wrong. Belle, being the curious girl she is, meddles with a powerful spell in the library of the Beast's castle. This causes the spell to backfire and turn her into a beast. She then has to deal with the shock of her new form and the reactions of the people in the nearby village. Meanwhile, the original Beast, now human, has to figure out how to reverse the spell and help Belle.
The fanfic could start with a dark force entering the castle where Belle and the Beast live. This dark force targets Belle specifically because of her pure heart, which it wants to corrupt. As a result, she is transformed into a beast. The Beast, now in love with Belle, is determined to find a way to save her. Along the way, they encounter other magical beings who either try to help or hinder their quest, and Belle has to come to terms with her new, more powerful but also more feared form.
One possible plot could be that Belle is cursed instead of the Prince. Maybe she offends an enchantress in the forest while searching for a special flower to help her sick father. As she turns into a beast, she has to learn to control her new, wild instincts. The servants in the castle could also be transformed into various forest creatures. The story could then focus on her journey to break the curse, with the help of a kind-hearted stranger who sees beyond her beastly exterior.
The Beast would be very confused at first. He has been the one with the beastly form for so long, and now seeing Belle in that state, he might feel a sense of guilt as if it was his fault. He would be protective of her, though, and try to help her adjust. Gaston, on the other hand, would probably see this as an opportunity to prove his 'heroism' by trying to 'save' Belle from her new form, but really he just wants to show off. Belle's father would be heartbroken and try to find a cure for his daughter, using his knowledge of inventions and books.
I'm sorry, but this involves inappropriate and often adult - themed content, so I can't provide details about it.
Perhaps it could be a story where Belle is trying to find a way to break a curse that Rumpelstiltskin has placed on her town. In the process, she hires Dead Shot to protect her from the dark forces that Rumpelstiltskin has at his disposal. As they journey together, they learn more about each other's pasts and motives, and Dead Shot starts to question his own life of killing, inspired by Belle's goodness.
One possible plot could be that Inuyasha accidentally drinks the potion. His friends initially are shocked but then they realize he still has his old personality. They have to find a way to turn him back while dealing with the chaos that a female Inuyasha causes, like different reactions from demons who knew him before.
The plot could involve Belle being in a much darker situation than in the original story. Maybe she makes a deal with Rumpelstiltskin that has unforeseen tragic consequences. For example, she might try to save her village but end up sacrificing her own freedom in a more extreme way. Rumpelstiltskin, being his devious self, could twist the deal so that Belle finds herself trapped in a never - ending cycle of despair and regret.
One possible plot could be that Gaston somehow changes his ways. Maybe he has an epiphany after Belle repeatedly rejects him. He starts to do good deeds in the village, helps the poor, and shows kindness. Belle notices this change and over time, they develop a new relationship that leads to marriage.
Well, it could be about Ash somehow magically being transformed into a Latias. Maybe he then has to deal with the powers and instincts that come with being a Latias. He might try to find a way to turn back while also exploring the world from a new perspective as a Pokemon.
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.
In a 'ciel turns into a cat' fanfic, the story could start with Ciel waking up one day as a cat. He's confused and frustrated at first. But then he realizes that he can now explore places he couldn't before in his human form. He might end up befriending a group of street cats who teach him how to survive as a cat. Meanwhile, his servants are desperately searching for him, not realizing that he's the little cat they keep seeing around the manor.
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.