Another possibility is a new dark wizard or witch who emerges and wants to prove themselves by taking on Harry and Ginny. This antagonist could be jealous of their fame and power and decides to target their DADA class to show that they are not as great as everyone thinks. They could try to undermine the students' confidence in Harry and Ginny's teaching.
There could also be some internal antagonists within the school. For instance, some old - fashioned teachers who don't approve of Harry and Ginny's unorthodox teaching methods. They might try to create problems for them, like trying to get them fired or changing the curriculum that Harry and Ginny have planned. This would add a lot of conflict to the fanfiction as Harry and Ginny would have to fight not only external threats but also internal opposition.
Well, it could be that Harry and Ginny use their own experiences with dark creatures and magic to create really interesting and hands - on lessons. For example, Harry might demonstrate how to fend off a Dementor using a Patronus charm, while Ginny could show how to deal with a Boggart in a practical way.
Obviously, Harry and Ginny are main characters. Harry, with his knowledge of the future events and his skills as a powerful wizard. Ginny, who is also a talented witch. And then, of course, the Marauders - James Potter, Sirius Black, Remus Lupin and Peter Pettigrew. They are the ones being taught, so they are also central to the story.
It could be a group of dark wizards who have escaped from Azkaban and formed a new dark alliance. They might be seeking revenge on Harry and Hermione for their role in defeating Voldemort.
One likely antagonist could be those who don't understand Cedric's possessiveness over Harry and see it as something abnormal or harmful. For example, some of Cedric's friends might think he's being too controlling and try to convince him to back off, which could create conflict. Another potential antagonist could be a character who has a crush on Harry and sees Cedric as an obstacle. They might try to break Cedric's hold on Harry, leading to a rivalry.
Voldemort or his remaining Death Eaters are often common antagonists. Even after his defeat in the original story, in dark fanfiction, they could come back in some form to target Harry and Ginny. For example, a Death Eater who escaped justice might seek revenge on Harry by harming Ginny.
It could be Voldemort or his remaining Death Eaters. Since they have always been the main antagonists in the Harry Potter universe, in this fanfiction they might still be the ones causing the danger that leads to Harry's sacrifice for Ginny.
It could be the corrupt prison guards or wardens. They might be in league with dark forces outside Azkaban. They oppose Harry and Ginny because the couple is getting too close to uncovering their illegal activities within the prison, like smuggling dark artifacts or collaborating with Death Eaters.
The obvious antagonists would be the Death Eaters. They are Voldemort's followers and are often causing trouble in the wizarding world. In this fanfiction, they might be the ones who have put Cedric in danger, either to lure Harry or for some other dark purpose.
One likely antagonist could be Draco Malfoy. He's from Daphne's social circle and might not approve of her being with Harry. He could try to expose their relationship out of jealousy or because of his pure - blood supremacist views.
Some pure - blood supremacists could also be antagonists. They may see Harry and Hermione, especially if Hermione is an heir as a Muggle - born, as a threat to their idea of a pure - blood - only wizarding society. They would do whatever it takes to stop them from rising to power and using the founders' legacy to promote equality and change in the wizarding world.