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.
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.
It could be some rogue Death Eaters who escaped justice. They might target Harry and Neville because they see them as a threat to their plans of a slow resurgence of the dark side. Since Harry is the Boy Who Lived and Neville was a key figure in the downfall of Voldemort, the Death Eaters would want revenge.
It could be that an old enemy of Stefan's family decides to target Bella as a way to get back at Stefan. This enemy might be another vampire clan or a supernatural being with a long - standing grudge. They would do everything in their power to disrupt the relationship between Bella and Stefan, causing all sorts of problems and conflicts for the two of them in the fanfiction.
A powerful dark wizard who has made a pact with a Titan. This antagonist could be using dark magic to corrupt the boundaries between the wizarding world and the world of the Greek gods. He wants to gain ultimate power by controlling both realms, and Harry and Percy would have to stop him.
This fanfiction might explore a scenario where Harry and Neville are 'sub mates', which could imply they are in a sort of subordinate or supportive relationship to one another. It could involve them facing challenges together within the wizarding world, like dealing with dark magic threats or school problems, with their relationship being a central aspect of the story.
It could be other vampires who are jealous of their unique relationship. Maybe there are some traditionalist vampires who don't accept this new kind of mating bond among them.
In some fanfictions, it could be the Volturi. They might see Bella and Alec's relationship as a threat to their power or their rules about vampire relationships. They could try to break them apart or manipulate the situation to their advantage.
One possible plotline could be Harry and Neville teaming up to discover a new spell or magical ability that has been long forgotten in Hogwarts. They might explore the Forbidden Forest together in search of ingredients for a potion related to this discovery.
The Volturi could be main antagonists. Since they have strict rules about vampires and humans, they might see Bella and Jasper's relationship as a threat to their order.
You could try looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. They have a large collection of Harry Potter - related fanfictions, so there's a good chance you'll find it there.