Perhaps it's related to Draco's involvement in certain dangerous or unapproved activities at school. Lucius might be warning him to stay away from things that could bring shame to the Malfoy name, like getting too close to Muggle - borns or half - bloods in a friendly way. In the wizarding world, the Malfoys are known for their pure - blood supremacist views, and Lucius would not want Draco to deviate from that.
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.
Well, usually it leads to some intense drama and conflict. Maybe Draco gets scolded or given some important instructions.
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.
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.
Well, in the fanfiction, Draco might be publicly shamed by the Howler. It could reveal some secrets he's been hiding, like his true feelings about his family's actions in the wizarding world. Maybe it scolds him for not living up to the pure - blood expectations.
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.
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.
He might be very embarrassed. Howlers are known for delivering their messages in a rather loud and public way. Draco, who is used to maintaining an image of superiority, would find it humiliating to have his family's business aired out in front of his classmates.
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.
Well, it could be that Draco did something to cross Annabeth in the fanfic's plot. Maybe he messed with one of her plans or said something disrespectful about her friends.
Perhaps Draco has been bullying some of Percy's friends or younger students that Percy feels responsible for. In fanfiction, the relationships can be tweaked a bit. Percy might be more hot - headed in some versions and not put up with Draco's pure - blood supremacist attitude towards those he deems as 'lesser' students. So he sends the Howler as a strong warning.