There could be a new character introduced who becomes an antagonist. Maybe a jealous student who is envious of Harry and Hermione's friendship or their magical abilities. This student could try to undermine them at every turn, from spreading false rumors to trying to get them in trouble with the professors.
Another possibility is that Voldemort's influence is still strong, and there are some dark forces within Hogwarts that are influenced by him. These could be in the form of cursed objects or hidden spies. Harry and Hermione would then have to use their intelligence and courage to root out these threats during their 6th year.
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.
The main antagonists could be the Death Eaters still loyal to Voldemort. They would be trying to undermine Harry and Ginny's efforts to protect Hogwarts and the wizarding world.
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.
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.
A new, made - up villain could be introduced. For example, a powerful dark wizard who has been in hiding for years and now sees Harry and Hermione as a threat to his plans. He could be after a particular magical power that Harry and Hermione unknowingly possess. He uses his dark magic to create all sorts of obstacles for them during their 5th year, from cursing their friends to setting up traps in Hogwarts.
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.
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.
Sometimes, the pure - blood supremacist families who oppose any changes to the traditional wizarding hierarchy can be antagonists. They see Harry and Hermione's attempts to take control and make things more equal as a threat to their long - held power and privilege. They may use their influence in the Ministry of Magic or other institutions to try and stop Harry and Hermione.
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.
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.