Perhaps in the fanfiction, Jack and Elsa find out about the baby during a time of peace between their worlds. They are overjoyed but also a bit scared. They start building a new home that combines the best of both their domains. As Elsa's pregnancy progresses, she discovers new aspects of her powers related to creation and warmth, which she uses to create a special nursery for the baby. Jack, on the other hand, starts learning about responsibility from the animals in his world. They both look forward to the day when their baby arrives and will change their lives forever.
The plot might involve Jack and Elsa initially being shocked at the news of the baby. Then, they start preparing for the new arrival. They could go on a journey to find a safe place to raise their child. Along the way, they meet different characters from their respective universes who either offer help or pose threats. Maybe Elsa struggles with her fear of accidentally harming the baby with her powers, while Jack tries to reassure her that they will figure it all out together.
One possible plot could be that Jack, with his carefree and fun - loving nature, and Elsa, with her strong and protective side, discover they are having a baby. They could face challenges like how to combine their two very different worlds. Jack's world of eternal winter playfulness and Elsa's more regal, ice - castle - based world. They might worry about how their baby will inherit their powers and whether they can keep their child safe from those who might want to use the baby's powers for evil.