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.
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.
You can try fanfiction websites like Archive of Our Own. It has a large collection of fanfictions, and there's a good chance you'll find some Caspian and Susan baby fanfiction there.
One popular Caspian and Susan baby fanfiction might be the one where they raise their baby in a magical forest. It shows their love for each other and how they pass on their values to the child.
The story could have a big impact on the magic in Narnia. Maybe the baby has a unique connection to the magic of the land, and its presence could either strengthen or disrupt the existing magical balance. Also, the idea of a new heir could inspire new quests and adventures, as various characters in Narnia would want to be part of the baby's life and future.
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.
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.