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, 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.
Often, when Draco gets a howler from Annabeth in fanfiction, it creates tension and conflict. It might expose something secret or force them to confront their feelings. Also, it could draw in other characters and complicate the story even more.
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.
There could be various plots. Maybe it leads to him changing his behavior or causes more conflicts within Slytherin. It might also draw attention from other houses and have consequences on the school dynamics.
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.
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.