One possible storyline could be that they face challenges in their relationship while expecting. Maybe they have to deal with outside forces trying to harm them or the unborn child. For example, some villains from their world might see the baby as a threat or a tool for their evil plans.
Their friends would probably be very excited. Annabeth might offer Piper advice on motherhood, based on her own experiences or knowledge. Percy could be protective of them, like he was with his own friends. He would be ready to fight off any threats.
Another theme might be the reactions of their friends and family. For example, how their demigod friends react to the news and how they all come together to prepare for the new arrival. In some fanfictions, there could be challenges related to their powers and how it affects the pregnancy. Maybe Piper's charmspeak or Jason's lightning powers have some unforeseen effects. And often, there's a focus on the emotional journey of Piper and Jason as they anticipate becoming parents, dealing with fears, hopes, and joys that come with it.
In fanfiction, Jason and Piper often have a complex relationship. They might start as friends, gradually realizing their deeper feelings. Their adventures together can strengthen their bond, and they support each other through various challenges. For example, in some fanfics, they face mythical creatures side by side, which makes them rely on one another more.
One possibility is that Piper has to face the challenges of single motherhood. She might struggle financially and emotionally at first. However, with the support of her friends, like Annabeth, she could learn to be strong and independent. She could focus on her career or other aspects of her life while raising the child on her own.
Perhaps Jason is scared. The idea of becoming a father might be overwhelming for him, and he doesn't feel ready to take on that responsibility. So, instead of facing it, he runs away.
You can try searching on fanfiction platforms like Archive of Our Own (AO3) or FanFiction.net. Just type in 'Piper and Jason pregnant' in the search bar on those sites, and you might find relevant stories.
Many Piper and Jason fanfictions focus on the theme of identity. Both characters have complex identities within the original story, and fanfic writers like to expand on that. For instance, Piper's heritage and Jason's forgotten past. Also, friendship is a theme. Their relationships with other characters in the fanfictions can be explored in different ways, such as how they support their friends during tough times. And don't forget the theme of sacrifice, as they are often in situations where they might have to give something up for the greater good.
The fanfiction might start with Jason and Piper falling asleep separately, but then their dreams merge. They find themselves in a beautiful, enchanted forest in their sleep. As they explore this new world together, they realize how much they truly love each other. Their love could be shown through small gestures like holding hands while walking through the forest, or by protecting each other from the strange creatures that lurk in the dream - forest.
In fanfiction, Jason and Piper's story can develop in many ways. One common path is their growth as individuals within the relationship. They learn from each other's strengths and weaknesses. For instance, Piper might teach Jason more about her cultural magic, while Jason could show Piper how to better strategize in battles.