It could be the Death Eaters. They might not want Harry and Hermione to succeed in the ritual as it could be a threat to their dark plans. Or maybe some other dark wizards who are envious of the power that the ritual might bestow on Harry and Hermione.
Perhaps there are some old -school pure -blood wizards who oppose the ritual. They think it's against the traditional ways of magic. They try to disrupt Harry and Hermione's efforts. Another possibility is a rival group of students from Slytherin who want to steal the glory of the ritual for themselves.
Well, perhaps the ritual is related to enhancing their magical abilities. Harry might be providing moral support and practical help like gathering rare ingredients for Hermione. Maybe they need to perform the ritual to protect Hogwarts from a new dark threat.
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.
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.
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.
The White Walkers could be major antagonists. They are a huge threat in Westeros and Harry and Hermione would have to use their unique magical abilities to combat them. Their cold - based magic and army of the undead would be a great challenge for Harry and Hermione.
Draco Malfoy is often an antagonist in such fanfictions. His family's beliefs in pure - blood supremacy could lead him to target Hermione, who is a Muggle - born witch. He might use his influence and his Slytherin friends to make her life difficult.
The wizarding media or gossip - mongers could be antagonists. If word got out about their relationship, it could cause a stir. They would try to expose Harry and Hermione's secret, putting pressure on their relationship. So they have to constantly be on guard against these prying eyes.
Maybe Voldemort's remaining followers. Even though Voldemort is gone, there could be some die - hard Death Eaters who are still bitter about the fall of their dark lord. They see Harry as the main reason for their downfall and Hermione as an easy target because of her Muggle - born status. So they plot against the newly - married couple, perhaps trying to hex them or steal their magic.
The Ministry of Magic itself could be an antagonist. If Fred and Hermione are running away because they've uncovered some sort of corruption or dark secret within the Ministry, the Ministry officials would want to stop them from spreading the truth.
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.