A storyline could be that Rapunzel's long hair has some special connection to the pregnancy. Maybe her hair glows brighter as the baby grows inside her, and Jack has to figure out how to protect this new magical aspect. Another idea is that Jack discovers he has some hidden powers that surface to protect Rapunzel during her pregnancy.
Maybe Rapunzel and Jack are in a world where pregnancy is a very different experience. For example, the baby might start showing signs of having unique abilities even before it's born. This causes a stir in their community. Some are in awe, while others are afraid. Jack and Rapunzel then have to fight against the prejudice and fear to ensure their baby is born safely and accepted in the world.
One idea could be that Jack and Rapunzel are living in a small, peaceful village when they discover Rapunzel is pregnant. They then have to face the challenges of preparing for a new life together, like building a bigger home. Another could be that they are on a magical adventure while Rapunzel is pregnant, and they meet different magical creatures who offer their help and wisdom for the upcoming baby.
A common theme is the anticipation and excitement of new life. Both Jack and Rapunzel are likely to be shown as being very eager for the baby to arrive. Another theme could be the challenges they face together as a couple during the pregnancy, like dealing with morning sickness for Rapunzel and Jack trying to be as supportive as possible.
One interesting fanfiction could be where Jack Frost accidentally freezes Rapunzel's long hair and then has to find a way to thaw it without causing her harm. Another might be a story where they meet in a magical forest during a solstice event and team up to defeat an evil sorcerer who is threatening both their worlds.
There could also be a more humorous storyline. The baby might be teething and keep chewing on everything in the castle, including Flynn's boots. Rapunzel tries different remedies to soothe the baby's gums while Flynn tries to keep the baby out of trouble. And as the baby grows, it starts to imitate Flynn's swashbuckling moves, which is both cute and a bit dangerous.
One possible storyline could be that Sam and Jack face challenges in their military careers while also dealing with the pregnancy. Maybe there are rules against their relationship in the military context, and they have to fight to be together and also ensure a safe and happy environment for the baby. For example, they might have to keep their relationship a secret for a while.
Well, perhaps they already live in a peaceful kingdom. Jack is a brave knight and Rapunzel, with her long hair, has special powers. When she becomes pregnant, Jack vows to protect her at all costs. He goes on quests to get special herbs for her health during pregnancy. Meanwhile, Rapunzel uses her powers to keep their home safe and they prepare for the arrival of their baby together.
A possible plotline could be that Jack Frost is on a mission in Rapunzel's world and accidentally stumbles upon her tower. They form an instant connection. As their relationship progresses, Rapunzel gets pregnant. Now, they need to figure out how to raise the child. Jack Frost wants to introduce the child to his icy world, but Rapunzel is worried about the cold environment. Meanwhile, there could be a villain, like an evil witch or a jealous magical being, who wants to harm Rapunzel and the baby. So they have to rely on their combined powers and the help of their friends from both their worlds to protect their family - to - be.
One more plot point could be that Barbie as Rapunzel discovers that her unborn baby has a special destiny. As she's pregnant, she starts having dreams about the baby's future. This could lead her on a quest to find out more about this destiny, and she might have to leave the tower in search of answers, all while being pregnant and facing many challenges along the way.
One interesting storyline could be Apple Jack's journey to find a rare apple variety that is said to have magical properties. This could involve her traveling to different parts of Equestria, facing various obstacles along the way, and making new friends. Another storyline might be her trying to teach the younger ponies in Ponyville about the importance of hard work and honesty, just like she learned from her family.
One possible storyline could be Marge getting pregnant again and the family's chaotic yet heartwarming preparation for the new baby. For example, Homer might try his best to fix up the baby's room but end up making a mess. Another could be Lisa imagining what it would be like to have a younger sibling and writing in her diary about her hopes and fears.