One interesting plot could be that Jack Frost discovers Elsa's baby has the power to control both ice and snow in a unique way. He then decides to mentor the baby in controlling its powers. Another could be that the baby's presence somehow brings a new season to the magical world that combines the best of Jack's winter and Elsa's Arendelle's climate.
One interesting Jack Frost and Elsa fanfiction could be about them teaming up to save their respective frozen worlds from a common threat. They both have powers over ice, so it would be a great adventure with them combining their skills.
Well, perhaps a power - hungry wizard from a neighboring magical realm. He hears about the baby's unique powers that combine Jack Frost's and Elsa's abilities and wants to harness that power for his own evil plans. Or it could be a group of dark fairies who are envious of the baby's special connection to both the winter world and Arendelle. They might try to kidnap the baby and raise it to be on their side, away from Jack and Elsa.
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.
In this fanfiction, a plotline could be about the baby's identity crisis. Since it is a product of two very different entities, it might struggle with where it belongs. Is it more of a human - like Elsa or a more ethereal being like Jack Frost? The story could show the baby's internal struggle and how Elsa and Jack Frost help it come to terms with its identity. Additionally, there could be a plotline about the baby's first encounter with magic from other sources, which could either be a threat or an opportunity for growth.
The story could be about the baby's discovery of its powers. It might start with the baby accidentally creating a small snowflake, which then grows into a large snowstorm. Elsa and Jack would then have to teach it how to control this power.
One plot idea could be that Jack Frost is drawn to Elsa's ice powers. He visits Arendelle out of curiosity. They could start by being a bit wary of each other but then realize they can learn a great deal from one another about controlling and using their powers in new ways. For example, Jack might show Elsa how to have more fun with her ice, like creating ice sculptures that can move on their own.
One idea could be that Elsa and Jack Frost discover they both have the power to control elements in a unique way. They could team up to save a magical world that is being threatened by a dark force that can nullify their powers. Another plot could be that they meet and initially clash because of their different approaches to using their powers, but then gradually learn from each other and fall in love.
A plot could revolve around Elsa and Jack Frost having to deal with the unexpected pregnancy during a time of unrest in their respective kingdoms. Maybe there are enemies who see the baby as a threat and try to harm Elsa. Jack has to use his powers to protect her and their unborn child. They could also encounter a wise old sorcerer who gives them advice on how to control and enhance the baby's powers even before it is born. As the pregnancy progresses, Elsa learns more about her connection to Jack's magic through their unborn child.
Well, it's possible that such a fanfiction exists. Elsa and Jack Frost are from different fictional universes (Elsa from Frozen and Jack Frost from Rise of the Guardians), but in the world of fanfiction, people often cross over characters and create all kinds of scenarios. However, I haven't personally come across a 'pregnant' fanfiction involving them. You might have to search on some dedicated fanfiction websites to find out for sure.
One possibility could be that their baby has unique powers. Since Elsa has ice powers and Jack Frost is also associated with ice and cold, the baby might be able to create the most beautiful ice sculptures with just a thought. The story could focus on how they raise this special child in a world that may not fully understand its powers at first.