Well, the Howler could have details of Voldemort's weaknesses that Harry has discovered. Maybe it would say something like 'You think you're so powerful, but I know your secrets, and I'm coming for you.' It would be a way for Harry to show he's not afraid and that he has the upper hand now.
The Howler might contain Harry's anger and defiance. He could be yelling about all the harm Voldemort has caused, like the deaths of his parents and many others. It could also include threats of how Harry will stop him once and for all.
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
It could lead to all sorts of chaos! Maybe Voldemort is furious and retaliates immediately.
Perhaps it leads to a confrontation between Harry and the fans, or it could inspire a change in the way the fans view Harry or Voldemort. Maybe it even sets off a chain of events that affects the entire magical world in the fanfiction.
Well, it could be that Harry sends a Howler to the Ministry of Magic. He might be frustrated with their inaction or wrong decisions regarding the safety of Hogwarts. The Howler could expose some of their secrets or simply be a very public outburst of Harry's dissatisfaction, leading to a big political mess within the wizarding world.
It could be Dumbledore. He was always one to oppose Voldemort and a howler from him would be a very interesting plot point. He might use it to taunt Voldemort or to try and make him second - guess his actions.
Maybe Harry was really angry and wanted to shock Voldemort. A howler is a bold way to show that anger!
Snape could also be a possibility. If Harry did something that Snape considered a serious offense in Potions class, like using an ingredient in the wrong way that could have caused an explosion, Snape might send a howler to scold him. However, it would probably also be filled with his usual sarcasm and dislike for Harry.
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.
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.
In the fanfiction, the howler could be from an unexpected source. Maybe it's from a long - forgotten enemy who has finally found a way to get to Voldemort. The howler's message could be a warning about a new threat that is coming his way, something that could even be more powerful than him. So Voldemort has to decide whether to ignore it or take it seriously.