The baby could be a very energetic and curious little one, always getting into things around the house, much to the amusement and sometimes exasperation of Harry and Susan.
One possibility is that the story could focus on how Harry and Susan adjust to their new life as parents. They might face challenges like sleepless nights and learning how to take care of the baby's every need.
Another option is to ask other Harry Potter fans directly. You can go to Harry Potter fan conventions or events (if there are any in your area) and ask around. People who are really into the fandom might know of some great 'Harry and Susan baby fanfiction' that they can point you towards.
In some 'Harry and Susan baby fanfiction', the focus could be on the baby's growth and how it impacts the relationship between Harry and Susan. For example, as the baby grows, they might have different ideas on how to educate the child. Harry, with his own difficult childhood, might want to be very protective, while Susan could have a more laid - back approach. This leads to some interesting discussions and growth in their relationship.
They could be two ordinary friends who have just decided that they need a change in their lives. Maybe they've grown up together in a small English town and now want to see what the world outside England has to offer.
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.
Their relationship might develop gradually. They could start as friends, sharing their post - war traumas and slowly realize they have deeper feelings for each other. Maybe they start spending more time together, going for walks in the Forbidden Forest or having tea in Hogsmeade.
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.