Maybe there could be a plot where an evil force in Narnia is trying to separate them. Lucy and Caspian have to go on a dangerous journey together to stop this force. Along the way, they rely on each other's strengths. Caspian protects Lucy from physical threats while Lucy uses her magic to solve various puzzles. Their bond grows stronger through these trials and they eventually confess their love.
You can focus on character development. Show how Lucy and Caspian grow as individuals within the relationship. For example, Lucy could become more confident in her magical abilities because of Caspian's belief in her.
One idea could be that Lucy discovers a hidden magical ability that helps Prince Caspian in a crucial battle. For example, she might be able to communicate with the spirits of the forest in a way that no one else can, and these spirits provide strategic advice to defeat an enemy. Another plot could involve Lucy getting lost in a mysterious part of Narnia and Prince Caspian embarking on a quest to find her, during which they both learn more about each other's pasts and grow closer.
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.