Perhaps the baby is born with a strong will and a sense of adventure. They embark on a journey to find out more about Jack's family background, as there was so little known about him in the original story. Along the way, they face challenges and make new friends, all while carrying the legacy of their parents' love.
Another idea is that the baby has some sort of special gift. For example, they can sense the emotions of the people around them, which is a connection to the deep love that Jack and Rose shared. The baby could use this gift to bring people together in the community, much like how Jack and Rose's love defied class boundaries.
One idea could be that their baby grows up with a love for the sea, just like their parents' fateful encounter on the Titanic. Maybe the baby discovers a hidden diary that details their parents' story and becomes inspired to preserve the memory of the Titanic in a new way, like through art or historical research.
Well, perhaps in the fanfiction, the pregnancy could be a catalyst for them to start thinking about their future more seriously. They might start planning a life together off the ship, looking for a place to settle down. It could also lead to some comical situations, like Jack trying to build a makeshift crib in their small cabin.
Another idea is that the baby girl is a new creation in Eyeless Jack's realm. She has unique powers that are slowly emerging. As she grows, Eyeless Jack takes on the role of a mentor, guiding her to control her powers and use them for good, or at least for their own survival in that strange and often cruel world.
In the fanfiction, Jack and Arcee could be magically merged into one being for a short time, and during that time, they create a new form of life. They then have to deal with the consequences of this unexpected creation and learn to be parents to this new being.
Maybe in the fanfiction, baby Jack Frost is a key to a powerful magic that Loki wants to control. But as he spends time with the baby, he forms an unexpected bond. They end up on an adventure together, facing off against other magical creatures who also want to get their hands on Jack Frost for his powers.
One idea could be that Elsa discovers she has the power to create a magical baby snow creature, and Jack Frost comes to visit and is fascinated by it. They then start to explore the possibilities of Elsa's powers together in relation to this new creation.
There could be a fanfiction where Jack somehow survives the sinking in a different way. Maybe he grabs onto a floating piece of debris that was overlooked. Then he and Rose are reunited on the rescue ship. They face challenges as Rose's family still doesn't approve of Jack, but they fight for their love and eventually get married.
There are fanfics that imagine different scenarios before they met on the Titanic. For example, a story where Jack was already an established artist in New York and Rose was introduced to his work before boarding the ship. This could change the dynamic of their relationship from the start.
One idea could be exploring the backstories of the characters in more detail. For example, Jennifer's life before coming to the orphanage. Another could be creating an alternate ending where the girls form a more positive and healthy community instead of the dark one in the game.
One interesting story could be where the Doctor and Rose have a baby with special time - traveling abilities. The baby's powers add a whole new dimension to their adventures, and they have to protect the baby from various threats across different time periods.
Perhaps in the fanfiction, Rose is pregnant and the Titanic doesn't sink. When they reach their destination, they have to figure out how to build a life together with a baby on the way. They face challenges like finding a place to live and Jack finding a stable job. But their love for each other and the anticipation of the new life keep them going.