It's all about imagining their adventures in a contemporary setting. You can have them dealing with modern problems while still retaining their dragon-training skills.
Often, the betrayal could stem from a secret Astrid keeps that accidentally harms Hiccup's interests or trust. Maybe she makes a decision without consulting him that has negative consequences for him.
First, focus on their relationship and character development. Make their interactions feel real and heartfelt. Also, incorporate exciting dragon adventures to add excitement.
The fanfiction about Hiccup and Astrid in 'How to Train Your Dragon 3' can vary greatly. Some might focus on their adventures, while others might delve into their internal conflicts and how they overcome them together.
To write such a fanfiction, focus on their emotions and chemistry. Create scenarios that heighten the tension and passion. Also, pay attention to character development to make it believable.
Well, in many fanfictions, the baby of Hiccup and Astrid brings new challenges and joys. Sometimes, the baby inherits their parents' bravery and spirit. But it varies a lot depending on the author's imagination and the story's direction.