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.
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.
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.
In this fanfiction, perhaps Stoick would be amazed by how much Hiccup has grown and changed in the future. He might see all the new inventions and ways of life that Hiccup has brought to Berk. Maybe there would be a moment of pride as he realizes his son has become a great leader.
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.
Well, writing a Stoick and Hiccup lemon fanfiction is a bit tricky. You should start with researching the 'How to Train Your Dragon' universe thoroughly. For the lemon part, you need to be very careful about how you handle the mature themes. You could start with a scenario where there's some sort of conflict or deep emotional moment between them that leads to the more adult - like interactions. However, be aware that this type of fanfiction might not be welcomed by all fans of the original work, as it can be seen as going against the family - friendly nature of the franchise.
I'm not entirely sure about specific Stoick and Hiccup lemon fanfictions. But in general, when looking for such fanfictions, you can try searching on some well - known fanfiction platforms. Just be cautious as the content can be quite adult - oriented in nature. It's also possible that there are more family - friendly interpretations that focus on their strong bond without the more inappropriate elements.
A possible plot is that there is a magical event that switches the minds of Stoick and Baby Hiccup. So Stoick has to figure out how to take care of himself as a baby while Hiccup, in Stoick's body, has to deal with the responsibilities of being the chief. It would be full of comical and heartwarming moments as they both learn about each other in a very different way.