In some rey falls to the dark side fanfiction, her turn could be due to a sense of betrayal. If she discovers some dark secrets about the Jedi Order or the Resistance, she might feel that the dark side is the only way to get true justice. For instance, if she finds out that the Jedi had some hidden, unethical practices in the past. Then she starts to embrace the dark side teachings and uses her powers in a more aggressive and self - serving way.
One possible plotline could be Rey being influenced by a powerful dark side artifact. Maybe she discovers an ancient Sith relic that slowly corrupts her mind and turns her to the dark side. She starts to have different views on power and the Force, believing that the ends justify the means.
One possible plotline could be that after facing great losses and betrayals, Leia starts to question the light side's values. Maybe the death of a close friend or the destruction of something she holds dear makes her vulnerable to the temptations of the dark side. She might then begin to explore the power that the dark side offers as a means to gain control and revenge.
Obviously, Rey is the central character as the story is about her fall to the dark side. Kylo Ren might also be a key figure. He could be either a catalyst for her fall, perhaps by tempting her with the power of the dark side, or he could be someone who tries to bring her back later on. Finn and Poe are also likely to be involved. They would probably be the ones trying to save Rey from the clutches of the dark side.
One possible plotline could be that Rey finally succumbs to the dark side under Kylo Ren's influence. He might train her in the ways of the dark side, leading them to rule the galaxy together with an iron fist. Maybe they start by taking over the First Order and using its resources to spread fear and chaos.
Kylo Ren might become more confident and powerful as he has Rey by his side in the dark side. He could see her as his equal and together they could develop a more complex relationship that is based on mutual understanding of the dark side's allure.
In some fanfictions, Luke might fall to the dark side after being betrayed by his closest friends. For example, if he discovers that they have been hiding important information from him that he thinks could have saved lives. This sense of betrayal could push him towards the dark side as he feels abandoned by the light. Also, a powerful dark side user could use mind control techniques on Luke, slowly turning him over time.
Obviously Rey herself is the central character. Kylo Ren often plays a role too. In some fanfictions, he might be the one who either tries to stop her from fully turning to the dark side because he has some sort of connection with her, or he might be the one who encourages her to embrace the dark side to be with him.
Another plotline might involve Kushina being brought back to life and getting to know Naruto as a young man. As she spends time with him, she discovers his determination and the way he has grown despite his hardships, which makes her develop feelings for him. For example, she could see how he interacts with his friends and how he's always there to protect them, and that could draw her to him.
One possible plotline could be Artemis having to deal with her new - found feelings while still maintaining her role as a goddess. For example, she might struggle between her duties and her attraction to Percy. Another could be Percy being completely unaware at first, leading to some comical misunderstandings. Maybe he thinks Artemis is just being overly friendly in a mentor - like way.
One interesting plotline could be their initial conflict gradually turning into a reluctant admiration. As they face common enemies, they start to see the good in each other. For example, Rey might notice Kylo Ren's internal struggle between the light and dark side, which makes her more empathetic towards him.