Well, Susan is often seen as a bit more worldly compared to Caspian in some fanfictions. Since she has been to Narnia before and has some knowledge of different cultures and magic there. Caspian, being from Narnia but in a different era, is in awe of her in a way. Their relationship can be complex. It could be a relationship where they both learn from each other. Susan can teach Caspian about the 'old ways' and Caspian can show Susan the new Narnia he is building. This kind of interaction makes for really engaging fanfiction.
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 possible fanfiction could start with Susan discovering she is pregnant. Caspian is overjoyed but also filled with a sense of responsibility. They start preparing a nursery in the castle, with Caspian bringing in the finest woods and fabrics. Susan spends her days knitting baby clothes, dreaming of their future child. Their friends in Narnia are excited too, and they all gather to offer their support and gifts. As the pregnancy progresses, they face some challenges, like when Susan has a difficult day of morning sickness, but Caspian is always by her side, bringing her soothing herbal teas.
Another theme could be the integration of Susan's human world values and Caspian's Narnian ways when it comes to parenting. This can lead to interesting cultural clashes and reconciliations. And of course, the theme of love is always present, not just the love between Susan and Caspian but also their love for their baby and how it shapes their actions and decisions in the story.
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.
Fanfiction should ideally be a celebration of the characters and their relationships in a respectful way. While 'lemons' fanfiction exists, it's not something that should be actively promoted. You can instead try to write your own Susan and Caspian fanfiction that focuses on their adventures, their friendship, or any other aspect of their relationship that you find interesting.
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.
I'm not sure about specific 'Susan and Caspian baby fanfiction' off the top of my head. However, fanfiction communities often have forums or groups dedicated to Narnia - related stories. You could join those and ask for recommendations. Maybe someone has already compiled a list of great fanfictions about this particular scenario.