Their relationship might start with curiosity. Hiccup would be curious about Elsa's ice powers and Elsa about Hiccup's connection to dragons. As they interact more, they start to respect each other's abilities and personalities. This mutual respect turns into friendship, and then gradually into love. The news of the baby would strengthen their bond as they now have a shared future to look forward to. They would support each other more, with Hiccup protecting Elsa during her pregnancy and Elsa using her powers to help Hiccup in his adventures.
Perhaps Hiccup is on a journey to explore new lands and stumbles upon Elsa's frozen kingdom. He helps Elsa overcome some of her fears about her powers that have resurfaced. In the process, they develop feelings for each other. Once Elsa becomes pregnant, they must deal with the cultural shock of their two very different worlds. Hiccup has to introduce Elsa to the Viking way of life, while Elsa tries to make Hiccup understand the elegance and traditions of her kingdom. They also have to figure out how their baby will inherit and control both dragon - related and ice - based powers.
Their relationship would likely become more solid. Having a baby together means they have a new shared responsibility. They would have to rely on each other more, like when taking care of the baby at night or making important decisions about the baby's future.
One possible antagonist could be a jealous sorcerer. He might be envious of Hiccup and Elsa's relationship and the power that their unborn child is said to possess. So he tries to harm Elsa during her pregnancy to stop the child from being born. Another could be some of the traditionalists from their respective kingdoms. They don't approve of their relationship and see the pregnancy as a threat to their old ways. They might try to separate Hiccup and Elsa, which would put Elsa's pregnancy at risk.
One possible plot could be that Hiccup, while on a journey with Toothless, somehow gets transported to Elsa's world. He stumbles upon Elsa as she is trying to control her powers in a difficult situation. Hiccup, with his experience dealing with dragons, offers her a different perspective on handling her powers. They gradually grow closer as they share their stories and in a moment of deep connection, they share a kiss.
Well, perhaps the fanfiction could be about how the baby brings Anna and Elsa even closer together. Since they already have a strong bond, the baby could be like a little miracle that makes their family complete. The story could show their daily life taking care of the baby, like singing lullabies and teaching the baby to walk, with a lot of cute and heart - warming moments.
One possible plot could be that Elsa and Hans, after many challenges and reconciliations, decide to start a family. Their baby might inherit Elsa's ice powers in a unique way, like being able to create ice sculptures just by touch. And they would have to protect the baby from those who might fear or want to use the baby's powers for their own gain.
If we're talking about a fanfiction where Elsa and Jack Frost have a baby, there are so many storylines that could develop. The baby could be a key to uniting different magical realms that are related to winter. It might face challenges growing up, such as being accepted in both Elsa's world and the world where Jack Frost belongs. The baby's powers could also be a source of both wonder and danger. For example, if it couldn't control its powers properly at first, it could accidentally cause blizzards or freeze things in inappropriate places. But with the love and guidance of its parents, it could learn to use its powers for good and become a beloved figure in the winter - themed magical world.
Perhaps the story could focus on how their friends react to the news. For example, Anna might be super excited and start knitting baby clothes, while Olaf could be confused about how a baby is made but still be very eager to be a playmate. The plot could also involve Elsa and Jack having to protect their unborn baby from some sort of threat in their magical realm.