What could be the content of the howler Harry gets from his parents in the fanfiction?

Well, it could lead to all sorts of drama and revelations. Maybe it's a scolding for something he did or a message of love and concern.

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.

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.

It could be Professor Snape. He often had a contentious relationship with Harry, and if Harry did something that Snape thought was particularly wrong or against school rules in a way that affected Snape's interests, he might send a Howler.

Harry could be angry at first. He might think his parents are overreacting or don't understand his situation at Hogwarts. But then, after thinking about it, he realizes they are just worried about him and starts to feel guilty.

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.

In this fanfiction, Harry might be publicly scolded by the Howler in a really embarrassing way. Maybe it reveals some secrets about him that he didn't want others to know, like his insecurities or small mistakes he made.

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 such fanfictions, it often leads to intense emotional moments and revelations. Harry might be shocked or confused initially.