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.
Perhaps Harry sends a Howler to an old enemy from his past, like Voldemort's remaining Death Eaters who are still causing trouble. This Howler could be a warning, full of Harry's newfound power and determination. It would show that he's not going to tolerate any more of their evil deeds and is ready to take action against them if they don't stop.
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.
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, Albus Severus Potter might be scolded severely in the howler. Maybe he got into some big trouble at Hogwarts, like getting caught cheating in a very important exam or accidentally releasing a dangerous magical creature in the school. The howler could be from his parents, especially if they found out about his misdeeds and were extremely disappointed.
Harry might be initially shocked and embarrassed. A Howler is a very public form of scolding, and in front of all his friends and classmates, it would be a humiliating experience for him.
Well, if Harry got a howler from Voldemort, it might be filled with dark magic and menacing words to unnerve Harry. It could also be a ploy to throw him off his guard and disrupt his focus on fighting evil.
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.
One possible reason could be that Harry has made a significant mistake in Potions class. Maybe he has endangered himself or others during an experiment, and Snape, being strict about safety and proper procedure, sends the Howler as a very public scolding.