Sometimes, in these fanfictions, it could be a rogue wizard who has a vendetta against Harry and Hermione. This wizard might be after a particular treasure in Gringotts and will stop at nothing to get it, even if it means harming Harry and Hermione. Also, there could be a power - hungry Gringotts official who wants to use Harry and Hermione for his own rise to power within the wizarding financial 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.
It could be a powerful dark wizard who has a grudge against the goblins and wants to destroy Gringotts. This wizard might have been wronged by the goblins in the past, and now he's seeking revenge by trying to bring down the bank. He could be using his own army of dark creatures to attack Gringotts.
The goblin elders who oppose Harry's new - found ownership could be the main antagonists. They might be worried about losing their power and influence within Gringotts to a human, especially one as famous as Harry Potter.
Voldemort or his Death Eaters could be antagonists. They might want to raid Gringotts to get powerful magical artifacts or gold to fund their dark deeds. Another option could be a group of corrupt wizards within the Ministry of Magic who see Gringotts as a source of wealth and power that they can control. They could try to undermine the Head Goblin and Harry Potter if they stand in their way.
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.
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.
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.