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.
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.
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.
His father, Harry Potter, might also send a Howler. Harry knows the importance of discipline and responsibility, and if James was shirking his duties or not following the rules at Hogwarts, Harry would not hesitate to use a Howler to communicate his disappointment and expectations.
Most likely, his parents, Harry and Ginny Potter, could send the howler. They would be concerned about their son's well - being and actions at Hogwarts. If Albus got into trouble or was not living up to the Potter family name, they would not hesitate to send a howler to set him straight.
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.
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.
It could be that someone did something really wrong and Harry needed to express his strong feelings in a very public and intense way. Howlers are notorious for being attention-grabbing and forceful.
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.
One possible plot could be that James Sirius Potter has done something really naughty at Hogwarts, like pulling a major prank that went too far. The Howler then arrives, screaming at him in front of all his friends and classmates, causing him great embarrassment. This could lead to him having to face the consequences and also learning a valuable lesson about the limits of his mischief.