There might not be a traditional 'big bad' antagonist. Instead, it could be the lake's own magic gone wrong. The lake has been corrupted over time, and its self - defense mechanisms target Hermione as she ventures too close. Harry has to figure out how to calm the lake's magic and save Hermione from its harmful effects without really having a sentient enemy to fight.
It could be some water - dwelling dark creatures. Maybe mermaids with a malicious streak or some sort of lake - bound demons that were awakened when Hermione got too close. They might see her as an intruder and want to harm her, which is why Harry has to save her.
Perhaps there is a dark wizard who has enchanted the lake. This wizard could be using the lake as a hiding place for something evil, and Hermione unknowingly stumbles upon it. The wizard then sends his minions, which could be shadowy figures or magically controlled animals in the lake, to stop Hermione from getting away. Harry has to face these minions to save her.
In this fanfiction, apart from Voldemort, there could be some dark wizards who are working independently but have the same goal of seeing the Potters dead. They might be jealous of the Potters' popularity or have some old grudges. These dark wizards could be in league with Voldemort or acting on their own, and they would be a big obstacle for Hermione as she tries to save the Potters.
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.
In some fanfictions, an ancient evil entity that has been awakened might be the main antagonist. This entity could be after Ginny because she has some special power or connection that the entity wants to use for its own dark purposes. And of course, there could be a jealous wizard who wants to get rid of Ginny to have Harry all to himself.
In this fanfiction, it could start with Hermione going to the lake for some research purpose. But things go wrong when she accidentally disturbs an ancient magic in the lake. Harry, who has a special connection to the magic around Hogwarts, senses her distress. He races to the lake and dives in. As he swims towards Hermione, he has to battle his own fears as well as the unknown threats in the water. Once he saves her, they realize that there is more to the mystery of the lake than they initially thought, and they team up to figure it out.
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.