One possible plotline could be that Lucy and Caspian are newly married and the pregnancy is a surprise. They then have to deal with the political implications in Narnia as well as their own personal fears and joys. Another could be that they face some sort of danger during the pregnancy, like an enemy threat that endangers Lucy and the unborn child.
I'm sorry, I don't have specific 'Lucy and Caspian pregnant' fanfiction to recommend directly. You could try searching on popular fanfiction websites like Archive of Our Own. There you can use the search filters to look for stories related to Lucy and Caspian with the theme of pregnancy. Just be sure to check the ratings and tags to find the ones that suit your taste.
One common theme is adventure. Given their backgrounds in the Narnia stories, fanfictions often send them on new quests. For example, they might explore uncharted parts of Narnia or other magical realms.
Fanfiction can provide a sense of continuation. Since their own story might have ended, fanfiction gives them the opportunity to be part of new tales, with new challenges and emotions. It also lets them explore aspects that weren't fully developed in the original story, like their deeper friendship or potential romantic feelings.
Romance is also a big theme. Writers like to explore the potential relationship between Caspian and Lucy. They might write about how their feelings for each other develop over time, from initial admiration to deep love. It could involve cute moments like sharing a secret in a hidden garden in Narnia.
A common theme is the 'forbidden love' aspect. Since they come from different worlds in the original stories, fanfictions often play on the idea that their love is something that goes against the norm or is difficult to pursue. Another theme is the 'growth through love'. Their relationship helps them both grow as individuals, with Lucy becoming more confident and Caspian becoming more open - hearted.
One common theme is adventure. Since they are characters from Narnia, the stories might involve them going on adventures while taking care of a baby. Maybe they are on a quest and have to protect the baby at the same time.
One common theme is the exploration of different cultures. Since Lucy is from one part of Narnia and Caspian has his Telmarine roots, their marriage often involves melding these two different ways of life.
Well, perhaps in the fanfiction, Lucy and Caspian are from different kingdoms that are at odds. But when they are sent as envoys to negotiate a peace treaty, they discover their mutual respect for nature and justice. They start to secretly meet and exchange ideas. As time passes, their feelings turn into love. They then have to struggle against the political forces that want to keep their kingdoms separate while trying to stay together.