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.
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.
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.
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.
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.
The Order of the Phoenix would be likely antagonists. Since Harry has turned to the dark side, they would see him as a threat to the peace and safety of the wizarding world. Dumbledore, if he were still alive in the fanfiction, would also oppose Harry. He would try to use his wisdom and power to bring Harry back to the light. Another antagonist could be Neville Longbottom. Neville, who has always been a symbol of resistance against the dark side, would not stand for Harry becoming a dark lord and would fight against him to protect Hogwarts and the values it stands for.
Dark wizards or witches who are on a power - hungry spree could be the antagonists. They might see Daphne as an easy target or a way to get to Harry Potter himself. So they kidnap her to lure Harry into a trap.
Voldemort or his Death Eaters could be antagonists. They might see the pregnancy as a way to manipulate or harm Harry and Hermione. For example, they could try to use the unborn child as a hostage or a weapon against them.
Apollo himself could be seen as an antagonist in a way, because of his possessive behavior towards Percy. His actions that restrict Percy's freedom and independence are antagonist - like. However, in some cases, other characters who oppose Apollo's possessiveness might also be antagonists from Apollo's perspective. For example, if Percy has a close friend who tries to protect him from Apollo, Apollo might view that friend as an enemy.
In Kidnaped Izuku Fanfiction, likely antagonists could be rival villains who see Izuku as a threat to their plans. For example, those who want to stop him from becoming a great hero. It could also be villains who are after his unique quirk. Maybe they want to study it or use it for their own evil purposes. Another possibility is that it could be a group with a vendetta against the hero school he attends, and they target Izuku to send a message.