The Ministry of Magic could be the antagonists. Since Harry is in a gang, they might see him as a threat to the order of the wizarding world. They could send Aurors after him and his gang mates, trying to arrest them and break up the gang.
Maybe there are some powerful and corrupt humans in the gang wars. They are not afraid of vampires and want to take over the territory that Edward's family has been protecting. These humans could be the main antagonists as they constantly create trouble for Bella and Edward.
The Dementors could be the main antagonists. They are always a threat in Azkaban, sucking out the souls and happiness of the prisoners. They would be constantly trying to get at Harry and Daphne.
It could be the corrupt prison guards or wardens. They might be in league with dark forces outside Azkaban. They oppose Harry and Ginny because the couple is getting too close to uncovering their illegal activities within the prison, like smuggling dark artifacts or collaborating with Death Eaters.
In some fanfictions, Voldemort or his remaining Death Eaters could still be the main antagonists. They might target Harry and Neville specifically because they see them as a threat to their plans of taking over the wizarding world again.
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.
It could be Voldemort or his remaining Death Eaters. Since they have always been the main antagonists in the Harry Potter universe, in this fanfiction they might still be the ones causing the danger that leads to Harry's sacrifice for Ginny.
Voldemort could still be an antagonist. He might see Harry's new - found relationship with Rowena as a threat to his plans. He could try to use the power of their love against them, or try to separate them to weaken Harry.
In many cases, the main antagonists can be other Slytherins who still hold grudges against Harry or are loyal to the pure - blood supremacist ideology. They might see Harry as a threat to their way of life. Also, some Death Eaters can be antagonists. They target Harry because of his role in Voldemort's downfall and would stop at nothing to get to him.
It could be a rogue Death Eater who escaped the clutches of justice after Voldemort's fall. This Death Eater might be seeking revenge on Harry and decides to target Bill as well since he's Harry's mate.
The primary antagonist is Sauron. His power and his armies are a constant danger to Frodo's mission to destroy the Ring. The Nazgûl, as his most feared servants, are also a major threat. They are not only powerful but also difficult to defeat. In addition, if the fanfiction includes elements from the Harry Potter universe, perhaps some Death Eaters or dark wizards who have found a way to cross over to Middle - earth could also be antagonists. They might side with Sauron in order to gain more power, seeing the potential in the Ring and the chaos in Middle - earth.