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.
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.
He might be initially shocked. Draco is used to being the one in control and having others be intimidated by him. A Howler from Percy, someone he may not have considered a huge threat before, would be unexpected. He could be angry and vow revenge in his mind.
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.
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.
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.
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.
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.