A plotline could involve them preparing for the arrival of the baby. This might include things like setting up a nursery, getting baby supplies, and dealing with their own fears and insecurities about becoming parents. It could also show how their relationship grows and changes during this time. For example, they might become closer as they support each other through the pregnancy journey.
One possible plotline could be the challenges they face in their relationship due to the unexpected pregnancy. Maybe they have to deal with the reactions of their friends and family.
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.
I'm not sure specifically about 'Jack and Tooth pregnant' fanfiction. You could try searching on popular fanfiction websites like Archive of Our Own. They have a vast collection of fanfictions, and you might be able to find something related by using relevant tags and keywords.
Perhaps the pregnancy is a symbol of a new era of peace between the world of Frozen and Jack Frost's domain. Elsa and Jack Frost have to work together to prepare for the arrival of the baby. They might build a new place that combines the ice and snow elements of both their worlds, and in the process, they face challenges from those who oppose the union, but they overcome them through their combined powers and the power of friendship.
One possible plotline could be a magical or technological mishap that makes it seem like Arcee could be pregnant. For example, a strange energy field affects her systems, and Jack, being her friend, helps her figure out what's going on. They go on a journey to find a way to reverse the effects, and along the way, they meet other characters who either help or hinder them.
One possible plotline could be that Tinkerbell's pregnancy is a magical accident. Maybe she was exposed to some strange magic that caused this unexpected situation. Another could be that she is in love with a fairy or a character from the Peter Pan universe and they are starting a family together.
One possible plotline could be how Nami and the other Straw Hat crew members react to her pregnancy. Maybe they would be overly protective or excited. Another could be about Nami dealing with the physical and emotional changes during pregnancy while still being part of the crew's adventures.
One possible plotline could be Ursula dealing with the challenges of pregnancy while still maintaining her powerful and devious nature. For example, she might use her pregnancy to her advantage in some scheme against her enemies.
A plotline could involve Mulan's partner and how they support her during the pregnancy. It could also show how Mulan's pregnancy impacts her relationships with the people she has met during her previous adventures. Maybe there are some who are skeptical or some who are very supportive. Also, the story could explore how Mulan prepares for the arrival of the baby in the context of the world she lives in, whether it's the military - based world she was a part of or her family - centered village life.
One possible plotline could be a character accidentally getting pregnant and then having to deal with the reactions of their friends and family. For example, in a 'Victorious' context, a main character like Tori might find herself in this situation and struggle to balance her normal life, like school and her performing career, with the new reality of pregnancy.
A possible plotline could be that at first, they are shocked by the pregnancy but then gradually accept it and start planning for their future together.