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.
Voldemort or his remaining Death Eater followers could also play a role. If they find out about Harry and Hermione's relationship, they might try to use it against them. For example, they could try to kidnap Hermione to get to Harry, or spread false rumors to create discord within the Order of the Phoenix.
Well, in some fanfictions, Voldemort or his remaining Death Eaters could be antagonists. They might see Harry and Hermione's relationship as a threat to their plans. For example, if they think that a united Harry and Hermione are more powerful and could foil their dark schemes more easily.
One possible plot could be that they have to keep their relationship a secret from Ron, who might be jealous. So they meet in hidden corners of Hogwarts, like the Room of Requirement, to spend time together.
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.
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.
One common theme is the secrecy aspect itself. Writers often explore how they keep their relationship hidden from their friends like Ron. Another theme could be the challenges they face due to the wizarding world's expectations. For example, there might be concerns about how it could affect their roles in the fight against Voldemort.
It could be other members of the royal family who see Harry as a threat to their power or position. They might try to undermine him and prevent him from claiming his rightful place as king.
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.
A great 'Harry Potter and Hermione dating fanfic' should have a believable relationship development. Since they were originally just friends in the books, it has to show a gradual shift in their feelings. For example, through shared experiences like facing a new dark wizard threat together.