One possible plot could be that the Howler is a warning about a dangerous magical experiment Hermione is about to undertake. It scolds her for not considering the potential consequences and warns her to stop. This would put Hermione in a difficult position as she has to decide whether to heed the warning or continue with her pursuit of knowledge.
Well, in the fanfiction, Hermione might be publicly scolded by the Howler in front of her friends. This could lead to her feeling extremely embarrassed. Maybe she made a mistake in her studies or violated some school rules in the fan - created scenario.
Well, it depends on the context set by the fanfiction author. It might trigger a conflict or a new plotline. Maybe it reveals something unexpected about Annabeth or changes the dynamics between the characters.
His mother could send the howler. In the wizarding world, mothers can be quite fierce when it comes to family reputation and she might be unhappy with Sirius's actions or friends.
It could be Severus Snape. He has a complex relationship with Dumbledore. Maybe Snape discovers some of Dumbledore's plans that he doesn't agree with regarding Harry Potter. He then sends a Howler in a fit of anger to express his disapproval.
Perhaps another student, like Hermione, could send the Howler in a very creative twist in the fanfiction. Maybe she has found out that Draco is planning something malicious against the Gryffindors and uses the Howler as a way to threaten him into stopping.
Hermione could be the sender. If Snape has made some unfair judgment in class against Gryffindor students, Hermione, with her strong sense of justice, might send a howler to him to voice her protest.
His mother is a likely sender. She could be worried about Remus and send a Howler to remind him to take his Wolfsbane Potion regularly or to be more cautious around others. Mothers are often protective, and in the wizarding world, a Howler is a rather forceful way to get your point across.
It could be Professor Snape. He often had a contentious relationship with Harry, and if Harry did something that Snape thought was particularly wrong or against school rules in a way that affected Snape's interests, he might send a Howler.
Hermione getting a howler in fanfiction could result in a range of reactions from her and those around her. It might cause conflict with her friends or make her more determined to prove herself. It all depends on the specific story and the author's imagination.