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.
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.
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.
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 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 might immediately gather his friends and share the contents of the Howler with them. He values their input and they've always been there to support him. Together, they would analyze the message and come up with a plan. Maybe he'd be a bit angry too, feeling that Voldemort is always trying to disrupt his life and the safety of those around him.
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.
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.