Their relationship would become more centered around the future. They'd have more serious conversations about how to raise the child. Valka might be more open about her past experiences with dragons to Stoick, so they can decide how to share that part of her life with the baby. And Stoick would be more cautious in his actions, not wanting to put Valka or the unborn baby in danger.
One idea could be exploring how they prepare for the new arrival. They might start renovating their living space, like adding a new room for the baby. Stoick could be shown as overprotective, not wanting Valka to take any risks. Another could be that they share their hopes and dreams for the child. Valka might hope the baby inherits her love for dragons, while Stoick hopes for a strong future leader.
It's hard to say if it's common. Since Valka is a fictional character, we don't really have data on her fictional reading habits. But in the context of the fandom, it could be seen as something that would be cool if she did, as it would show her interest in the fan - created content.
Unique? Well, it might be the way it can explore their relationship in a more...intense way. But since 'lemon' often involves mature themes, it can be a bit controversial. It might show a side of their bond that is different from the regular family - friendly portrayals in the original work.
One common theme is the growth of Hiccup under Stoick's guidance. Even if they have differences, Stoick's wisdom and experience help Hiccup become a better leader. Another theme is sacrifice. Stoick might sacrifice for Hiccup, or Hiccup for the ideals Stoick instilled in him.
Since I'm not familiar with the specific 'valka erotic story', it's hard to say if there is a non - adult interpretation. But often, if a story has an adult theme, it can be reimagined in a more family - friendly way by focusing on other aspects such as the characters' relationships in a non - sexual way.
One common theme could be the anticipation and excitement surrounding the new life. It might explore how Valka and those around her prepare for the baby.
The main characters are clearly Stoick and Hiccup. Stoick is Hiccup's father, and in this fanfiction, their encounter forms the central plot. Hiccup, as the character from the future, has likely developed and changed a great deal since Stoick last saw him.