In the fanfiction, the Howler could be a warning from an unknown dark force. Dumbledore has been making moves to counter the rising dark magic in the wizarding world. This dark force, not wanting to be thwarted, sends a Howler as a form of intimidation. The Howler could contain veiled threats and references to past events that only Dumbledore would understand.
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.
The plot might revolve around the mystery of who sent the howler. It could be an enemy trying to intimidate Molly or perhaps a misdirected howler that was meant for someone else with the same name. In the story, Molly then has to figure out the true sender and their intentions, which could lead her on an adventure.
Well, perhaps the howler could be from Lily. It might scold Snape for his actions in siding with the Death Eaters in his youth. It could be a moment of self - reflection for Snape as he listens to the angry tirade from someone he once deeply cared about.
In the fanfiction, the howler could be from an unexpected source. Maybe it's from a long - forgotten enemy who has finally found a way to get to Voldemort. The howler's message could be a warning about a new threat that is coming his way, something that could even be more powerful than him. So Voldemort has to decide whether to ignore it or take it seriously.
In this fanfiction, perhaps the howler scolds Harry for something he did that he thought was right but others disagreed. Maybe it's about his actions during a Quidditch match that were considered too risky. The howler could be from McGonagall, for example.
In the fanfiction, the Howler Teddy Lupin gets could be a case of mistaken identity. Some powerful magic could have misdirected the Howler meant for someone else to him. Teddy then has to figure out who the real recipient is while also dealing with the public humiliation of the Howler's scolding. He might start an investigation at Hogwarts, teaming up with his friends to solve the mystery and clear his name.
The fanfiction could focus on Dumbledore's backstory. Fans might write about his early years at Hogwarts as a student, his friendships, and how he developed into the great wizard he became. It could also include his interactions with other magical creatures in more depth than the original series, such as having secret meetings with centaurs or merpeople to gain wisdom.
One possible plot could be that Dumbledore is framed for a crime he didn't commit. Maybe there's a dark wizard who has managed to plant false evidence against him. For example, evidence that he was involved in some sort of dark magic plot. Another could be that the Ministry of Magic, fearing his power and influence, decides to arrest him on false pretenses to gain more control over Hogwarts. This could lead to a lot of chaos in the wizarding world as Dumbledore has many allies and supporters who would not stand for his unjust arrest.
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.
One possible plot could be that James Sirius Potter has done something really naughty at Hogwarts, like pulling a major prank that went too far. The Howler then arrives, screaming at him in front of all his friends and classmates, causing him great embarrassment. This could lead to him having to face the consequences and also learning a valuable lesson about the limits of his mischief.