Maybe the Howler contains threats about being disowned or removed from the family tree. In fanfiction, the Black family is very much into their pure - blood status and Sirius's actions that seem to side with Muggle - borns and blood traitors would be seen as a great disgrace. His mother could be yelling in the Howler about how he's ruining the family name and all the privileges that come with being a Black.
Well, it could be scolding him for his association with the 'wrong' sort of wizards, like the Potter family perhaps. His mother might be furious about his choices that go against the pure - blood supremacist views of the Black family.
Perhaps Sirius is scolding Harry for getting involved in something risky, like sneaking into the Forbidden Forest. He might be worried about Harry's safety and uses the howler to convey his strong emotions. In the fanfiction world, Sirius could also be using the howler to give Harry some crucial information about his family that he has just discovered and wants Harry to know right away.
Often, when Sirius gets a howler from his mother in fanfiction, it triggers a series of events. It might make him rebel more, or it could cause him to reflect on his actions. Sometimes, it even sets up conflicts with other characters.
It could be about Draco's performance at Hogwarts. Maybe Lucius is disappointed in Draco's grades or his behavior towards other pure - blood families' children.
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.
Well, it could be about him getting into trouble at Hogwarts. Maybe they scold him for sneaking out at night or getting involved in a dangerous adventure without permission.
In such a fanfiction, Sirius could receive a howler that reveals some family secrets. The howler might be a way for his family to try and control him or bring him back into the fold. It could also lead to some internal conflict within Sirius as he has to deal with the public humiliation of the howler and what it says.
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.
His father, Harry Potter, might also send a Howler. Harry knows the importance of discipline and responsibility, and if James was shirking his duties or not following the rules at Hogwarts, Harry would not hesitate to use a Howler to communicate his disappointment and expectations.
In some fanfictions, Sirius might use it as a chance to show off his independence. He could read the Howler out loud in front of his friends, laughing at his mother's old - fashioned views. This would show that he doesn't care about her threats and is proud to be different from the rest of the Black family.
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.