In this fanfiction, they could go on a journey across Narnia to discover hidden secrets of the land. Since they are now married, they have a deeper connection and can communicate without words in some magical ways. Along the journey, they find ancient spells and artifacts that help them better understand the history of Narnia and how to protect it for future generations.
It could be a jealous sorcerer who was once in line for the throne of Narnia. Now that Prince Caspian has married Susan, he sees his chance of power slipping away. So he tries to create chaos and turn the Narnian people against the new couple.
A common theme is the cultural differences they might face. Since they come from different worlds in the original stories, fanfiction often explores how they bridge those gaps in their married life.
One great source could be fanfiction.net. There are often many different takes on Caspian and Susan's relationship in fanfiction on that platform. You can search using the tags related to their names and'married' to find relevant stories.
One idea could be that Susan returns to Narnia during a time of great peril, and she and Prince Caspian have to team up to defeat a new, powerful enemy. They might discover hidden powers within themselves along the way.
The relationship can be developed through shared adventures. For example, they could be on a mission to retrieve a lost magical artifact. During this journey, they rely on each other's skills and start to respect and admire one another more deeply.
One possible theme could be love and commitment. The pregnancy would force them to reevaluate their relationship and show how deeply they care for each other. Another theme might be the impact on Narnia. As Caspian is a prince, there could be political implications of this union and the upcoming child. Also, there could be a theme of sacrifice, like Susan might have to sacrifice some of her own desires or plans for the sake of the baby and Caspian.
One idea could be that Susan returns to Narnia in a time of great need, and she and Prince Caspian have to work together to defeat a new, powerful enemy. Maybe they discover an ancient magic that only they can unlock through their combined efforts.
By having them go on adventures together. As they face perils like battles against dark creatures or solving mysteries in Narnia, they'll learn more about each other and their relationship can grow.
I'm not sure specifically if there is a fanfiction with Susan and Caspian where there is a pregnancy. There are a vast number of fanfictions out there, and it would depend on the creativity and direction of different authors. Some fanfictions might focus on adventure and relationship building between the two characters without including such a plot element, while others might explore more complex storylines that could involve pregnancy.