The various power - hungry lords and ladies in Westeros might also be antagonists. They could see Harry and Hermione as a threat to their power or a tool to be used. For example, Cersei Lannister might try to manipulate them to her own ends, and if they resist, she would become their enemy.
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.
One possible plot could be that Harry and Hermione somehow end up in Westeros through a magical portal. They might use their magic skills from the wizarding world to deal with the political intrigue in Westeros. For example, they could help a certain house in the Game of Thrones - like scenario, using spells to protect their allies or uncover the secrets of their enemies.
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 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.
Often, it could be Death Eaters. They are the main villains in the Harry Potter series, so it makes sense for them to be antagonists in fanfiction where Harry and Hermione need to escape.
Often, the goblins can be antagonists. If they have their own motives that conflict with Harry and Hermione's, like protecting a dark secret in the vaults or trying to take advantage of the situation for their own gain. Another possible antagonist could be a Death Eater who has infiltrated Gringotts to either steal something important or to ambush Harry and Hermione.
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.
Another possibility is that there could be some corrupted magical authorities who don't want Harry and Hermione to be in Forks for some political reasons. They could send their minions to create trouble for Harry and Hermione, making them the main antagonists in the fanfiction.
It could be the Ministry of Magic if they turn corrupt or power - hungry. They might see Harry and Hermione as a threat to their authority and try to stop them at all costs. Another possible antagonist could be a dark wizard or group of dark wizards who are using the chaos in the wizarding world to their advantage and Harry and Hermione are the only ones standing in their way.
Often, it could be dark wizards or witches who are envious of Hermione's status as the heir. They might want to take over Hogwarts themselves or steal the power that comes with being the heir. For example, a powerful dark wizard who has been in hiding for years might emerge to challenge Hermione.
In some fanfictions, it could be the Death Eaters still causing trouble. They might be targeting Harry and Hermione specifically during their 6th year, trying to disrupt their studies or even harm them.