Perhaps a scene where Hermione confronts Snape about his treatment of her friends. In the fanfiction, she could gather evidence of how Snape has been singling out Harry or Ron, and then she barges into his office. She would start off calmly, but as Snape tries to dismiss her, she gets more and more passionate, really telling him off for his behavior.
Hermione's character development in these fanfictions can be complex. When she tells Snape off, she may face consequences that force her to grow in different ways. For instance, Snape could try to retaliate by making her academic life difficult. This would make her learn how to deal with adversity and still stay true to her principles. She might also start to question some of the school's rules and traditions, as she realizes that someone in a position of authority like Snape can misuse them. This leads to her becoming more of a rebel and a reformer within the Hogwarts community, which is a significant development from her initially rule - abiding self.
Hermione is the central figure here as she is the one driving the action. She is likely to be portrayed as a bit more feisty and assertive than in the original books. Snape is also a main character as he is the target of Hermione's outburst. However, other characters like Harry and Ron might also make appearances. They could be the ones who witness Hermione's confrontation with Snape or even be involved in the events that lead up to it. For example, Harry might tell Hermione about something unfair Snape did, which then prompts her to act.
One possible secret could be about his past relationship with Lily Potter. Maybe he reveals how much Lily meant to him and the complex feelings he had towards her, which in turn influenced his actions throughout the series. Another secret could be related to certain spells or potions that he has discovered but kept hidden from others in the wizarding world.
One reason could be Hermione's excessive worrying. She often frets over small details and Harry might get tired of it and tell her off. For example, if she keeps nagging about the proper way to perform a spell when they are in a hurry during a dangerous situation.
One possible plotline could be that Hermione stands up to Snape for constantly being unfair to Gryffindor students. Maybe she discovers some misinformation he has spread about her friends and confronts him publicly in the Great Hall, causing a stir among the students and teachers alike.
One possible plot could be that Hermione tells off Molly for being overprotective of Ron. Maybe Molly is constantly interfering in Ron and Hermione's relationship, and Hermione finally has enough and gives her a piece of her mind about respecting their privacy and decisions as adults.
In this fanfiction, it could be that Ginny is involved in some sort of gossip or spreading rumors. Hermione, who values truth and integrity, confronts Ginny about it. She might scold Ginny for being irresponsible with her words and how it can harm others' reputations at Hogwarts.
Maybe Hermione has been too pushy with her academic knowledge in the fanfiction. Harry, who might be feeling stressed or frustrated, tells her off for not considering the emotional and practical aspects of a situation. He could say she's being too much of a know - it - all and not listening to others' ideas.
Hermione would likely become more empathetic towards Snape. Knowing his secrets would make her see him in a new light, not just as the strict and often - disliked professor. She might start to understand the reasons behind his actions and his prickly attitude.