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.
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.
It could be the remaining Jedi. Since Rey is turning, the Jedi who still uphold the light side would see her as a threat. They would try to stop her from causing more harm with her newfound dark side powers.
In this fanfiction, it could be that Rey has some personal losses that make her turn. She might lose someone dear to her and in her grief and anger, she decides that the dark side can give her the strength to prevent such losses in the future. She then starts learning the ways of the dark side, which changes her relationships with her friends.
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.
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.
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.
The consequences would be huge. The Resistance would lose one of its most powerful allies. It would also mean a new threat to the galaxy. Rey's turn to the Dark Side could inspire others to follow her, leading to a resurgence of the Sith. The balance of the Force would be severely disrupted.
One possible plotline could be Windu's internal struggle as he is gradually lured to the dark side. Maybe he starts to question the Jedi Code more and more as he experiences certain events or is influenced by a powerful dark side figure.
It could be a new, powerful dark side user created specifically for the fanfiction. This character might have a vendetta against the Skywalker family and sees turning Leia as a way to get revenge on Luke and Anakin. They could use Leia's insecurities and her desire to protect her loved ones against her.