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.
Perhaps a powerful dark wizard from a far - off land who has heard of Harry's fame and wants to prove himself by defeating Harry. Bill gets involved as he tries to protect Harry.
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.
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.
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.
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.
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.
One common theme is loyalty. Since they are mates, loyalty is a big part of their relationship. In many fanfictions, they will stick by each other through thick and thin, no matter what challenges come their way.
One interesting plot could be that Harry and Bill go on a dangerous adventure together in the Forbidden Forest. They might encounter a new and mysterious magical creature that they have to figure out how to deal with.
There could be. However, this is a rather unusual combination for a soul mates concept in the Harry Potter universe. It really depends on the creativity of the fanfiction authors. Some might take a more dark and angsty approach with Voldemort involved, while others could make it a more comical or out - of - the - box story with Bill and Harry.
Well, obviously Harry is one of the main characters. His friends like Ron and Hermione could also be important characters in 'Harry Mates' fanfiction as they are often his closest mates in the original series.