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.
It could be Dumbledore. He was always one to oppose Voldemort and a howler from him would be a very interesting plot point. He might use it to taunt Voldemort or to try and make him second - guess his actions.
Perhaps it's a taunt about Harry's power. Voldemort could be saying that all his efforts to defeat him are in vain because Voldemort is far more powerful. He might mock Harry's attempts at learning new spells and defenses, saying they won't be enough to stop him in the long run.
It could lead to all sorts of chaos! Maybe Voldemort gets really angry and lashes out.
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.
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.
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 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 Howler might contain Harry's anger and defiance. He could be yelling about all the harm Voldemort has caused, like the deaths of his parents and many others. It could also include threats of how Harry will stop him once and for all.
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.