The contrast between their personalities. Hiccup is more calm and calculating while Dagur is wild and impulsive. When a fanfic plays on this difference, it can create really engaging storylines.
Hiccup could try to resist by using his intelligence. He might find loopholes in Dagur's plans or demands. For example, if there are certain cultural or traditional rules that Dagur is using to force the marriage, Hiccup could find ways to show that those rules are being misinterpreted or misapplied.
The unique character dynamics. A female Hiccup would bring a different perspective to the relationship with Dagur. Their interactions can be full of surprises and new interpretations of the original story's relationships.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a vast collection of fanfics on various topics, including those about Female Hiccup and Dagur. Just use the search function and type in relevant keywords.
The relationship can be developed through shared adventures. For example, they could go on a quest to find a rare dragon. Along the way, they face difficulties like harsh weather and dangerous terrains. By relying on each other's skills, they start to bond. Dagur's strength and combat skills combined with Female Hiccup's intelligence and dragon - handling abilities make them a great team. This mutual dependence can lead to deeper feelings.
In such a fanfiction, it could be a really dark and complex story. Maybe Dagur uses threats or some form of blackmail to force Hiccup into this situation. Hiccup, being the strong - willed character he is, might initially resist and try to find a way out. However, if he has no other choice, he could be forced into a marriage that goes against his true feelings. This could lead to a lot of internal conflict for Hiccup, and also a strange and tense relationship between him and Dagur.
One plot idea could be that Dagur and Female Hiccup are forced to team up to face a new threat in their land. They start off not trusting each other but as they go through various challenges together, they begin to respect and then develop feelings for one another. Another could be a story where Female Hiccup has to save Dagur from a dangerous situation and in the process, Dagur realizes how strong and amazing she is, leading to a friendship that could turn into something more.
Well, start by imagining unique scenarios for Dagur and Hiccup's interaction. Maybe have them on a dangerous adventure together or dealing with a common enemy.
Typically, when Dagur kidnaps Hiccup in the fanfiction, it creates a huge crisis. It could be for revenge, to gain some sort of advantage, or just out of pure malice. This event often forces the characters to face their fears and grow stronger.