There could be an evil spirit from another dimension that is attracted to the new power that Prince Caspian and Susan's union has created. This spirit wants to possess one of them to gain control over Narnia. It starts by causing nightmares and spreading fear among the Narnian people to weaken their trust in the new rulers.
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.
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.
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.
The White Witch could make a great antagonist even in a Prince Caspian and Peter fanfiction. Her evil magic and desire for power are well - known. She could return in some form, perhaps a remnant of her magic that was left behind and has now taken on a new, more insidious form. Another option could be a power - hungry Narnian noble who wants to overthrow Prince Caspian and sees Peter as a threat to his plans. He could be sly and manipulative, causing trouble for both of them.
One possibility could be that the story focuses on how they adjust to parenthood in the magical world. They might face challenges from those who oppose their union or from the powers that be in Narnia. The baby could also have some special powers or a great destiny.
A plot could involve Caspian being cursed and Susan being the only one who can break it. She has to go on a quest to find a magical object. Caspian, in his cursed state, can't directly help but his spirit encourages her. Along the way, Susan discovers her own hidden powers and in the end, she breaks the curse and they confess their love for each other.
A possible plot is that Susan discovers she is pregnant during a time of unrest in Narnia. Caspian is worried about how to protect her and the unborn child. They decide to go into hiding for a while in a secret part of Narnia. There, they live a simple life, and Caspian learns to be more self - sufficient, like fishing and hunting for food. As the pregnancy progresses, they form a deeper bond. Eventually, they return to the kingdom when things have calmed down, and they are welcomed with open arms as they await the birth of their child.
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 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.