Who might be the main antagonist in a 'Harry Potter makes a Horcrux' fanfic?

It could be Dumbledore. In this fanfiction, Dumbledore might see Harry making a Horcrux as a threat to the wizarding world and tries to stop him at all costs. He could be the main antagonist because he stands in the way of Harry's actions, even if Harry has what he believes are good reasons for making the Horcrux.

Harry Potter would surely be one of the main characters. Since it's about the last Horcrux, he's the one on the mission to destroy it. His friends, like Hermione and Ron, would likely also be main characters as they usually accompany Harry on his adventures.

Well, perhaps in the fanfic, Harry discovers a new form of magic that he misinterprets as a way to gain an edge over Voldemort. He thinks that by making a Horcrux, he can store part of his soul safely away so that if Voldemort tries to kill him, he can come back. He chooses his wand as the Horcrux object. But as he does it, he starts to feel the darkness seeping into him and realizes he has made a terrible mistake.

Dumbledore could also play a role, even if it's from beyond the grave. Maybe he left behind some clues or instructions that Harry only discovers when he's in the middle of trying to remove the Horcrux.

Perhaps his father's old wand. It has a connection to his family and his heritage. Since it's a wand, it already has a certain amount of magical energy, so it could potentially be a suitable vessel for a Horcrux. And it would be a very interesting twist on the concept of a Horcrux as wands are not a typical choice.

Another possibility is that the wizarding legal system or a corrupt official within it could be the main antagonist. They might be trying to make things difficult for Harry, or perhaps they are in cahoots with the Weasleys and are trying to cover up the Weasleys' misdeeds. This would add an interesting layer of complexity to the fanfic.

It could be a new dark wizard who is jealous of Harry's ability to lift Excalibur. This wizard might want the sword for himself to gain ultimate power. He could start causing chaos in the wizarding world, trying to draw Harry out and take the sword from him. He might even team up with some of the Death Eaters who are also interested in the power the sword could bring.

Maybe the main antagonist is a dark force that has been slowly taking over the alliances from within. Harry notices this and decides to cancel the alliances to prevent the dark force from spreading further. This dark force could be something like an ancient evil that has awakened and is using the alliances as a way to gain a foothold in the wizarding world.

Perhaps there's a member of the royal court who is secretly envious of Harry's power and tries to undermine him. This person could be plotting against Harry and trying to make the queen believe that he is a danger rather than an asset. They might spread false rumors or create false evidence to get rid of Harry.

Yes. In the Harry Potter fandom, the concept of Harry being a Horcrux is quite a popular and fascinating one for fanfic writers. It adds a whole new layer of complexity to the story and character relationships.