One possible plot could be that they team up for a magical tournament that's different from the Triwizard Tournament. They discover they both have a passion for a rare branch of magic and decide to research it together, facing various challenges along the way.
In this fanfiction, their different magical abilities could be explored in a friendly competition. They could have a series of magical challenges where Harry uses his Gryffindor - like courage and quick thinking, and Viktor uses his Durmstrang training in elemental magic. Through this competition, they not only show off their abilities but also learn from and respect each other's skills.
Hermione's friends from Hogwarts like Harry and Ron might be a bit shocked at first. Ron, in particular, might have some mixed feelings as he has had a bit of a competitive streak with Viktor in the past. But eventually, they would come around and support her. Viktor's friends from Durmstrang would probably be more accepting as they might respect his choice more blindly. They could be excited for him and offer their own unique magical gifts or knowledge for the upcoming baby.
Hermione's friends would have a range of reactions. Ginny might be sympathetic towards Hermione, understanding the difficult position she's in. Neville could be worried about how this would affect Hermione's studies and her role in the fight against the dark forces. For Viktor's friends, they might try to encourage him to make the best of the situation. His Quidditch teammates might see it as an opportunity for Viktor to show his maturity and handle a complex situation. However, some of his closer friends might also be worried that he's getting involved in something that could end badly for him.
A prevalent theme is mutual respect. Fanfictions often show how they come to respect each other's skills and abilities. Also, there is the theme of cooperation. They might join forces to face a common enemy or solve a problem. For instance, in some stories, they work together to stop a dark wizard who is causing trouble in both their wizarding communities. In addition, the theme of personal growth can be seen as they influence each other's development as wizards and as individuals.
The friendship between Harry and Viktor in fanfiction might also be influenced by their relationships with others. Viktor may befriend Harry because he sees Harry's loyalty to his friends, like Ron and Hermione. Harry, on the other hand, could be intrigued by Viktor's mysterious nature. They could start by sharing small conversations, perhaps about their families or their dreams, and gradually grow closer as they find more in common.
Maybe Viktor has been cursed in some way in the fanfiction, and his actions towards Hermione are involuntary. Harry, being observant, picks up on the strange behavior. He may call upon his friends for help, like Ron. Together, they could form a plan. Harry might use his invisibility cloak to get close to Viktor and Hermione without being noticed and then use a powerful stupefy spell to stop Viktor before he can harm Hermione. And then they would figure out how to break the curse on Viktor.
One common theme is the discovery of their soulmate bond. Usually, they start off not knowing about it and then gradually realize their special connection.
Overcoming differences is also a big theme. They come from different wizarding cultures, so in many fanfictions, they have to work through those cultural gaps as they become mates. For example, different magical traditions and social norms in their respective countries.