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.
One possible plot could be that they face challenges in Narnia while Susan is pregnant. Maybe there are threats to the kingdom that they have to deal with together, all while preparing for the arrival of their child. Caspian might be extra protective of Susan, and they could rely on the help of their friends in Narnia.
Susan could develop into a more nurturing figure. Since she's pregnant, she might become more in tune with the needs of others in Narnia, not just her own family. Caspian, on the other hand, might grow in his sense of responsibility. He now has a family to protect, so he may make different decisions regarding the kingdom.
One interesting 'Prince Caspian' fanfiction could be about Caspian's adventures before he met the Pevensies. Maybe it details his training as a prince and how he learned about Narnia's secrets.
The main themes include adventure. The whole story is filled with various adventures from battles to exploring new parts of Narnia. Friendship is also key. The relationship between the different characters, whether it's between the Narnians or between the Pevensie children and the Narnians, is based on friendship. And there is a theme of magic. Narnia is a magical world and the use of magic and the presence of magical creatures play an important role in the novel.
Prince Caspian is of course a very important character. He is the rightful heir to the throne of Narnia. The Pevensie children - Peter, Susan, Edmund and Lucy - are also crucial. They are summoned back to Narnia to help Caspian. Trumpkin, the dwarf, is an important character too. He is a bit grumpy but very loyal and helps the group in their adventures.
The theme of friendship and alliance is also prevalent. Hermione and Prince Caspian usually form an alliance. They work together towards a common goal, whether it's saving Narnia from a new threat or bringing about a better future for the land. Their friendship grows as they rely on each other's strengths and overcome their weaknesses.
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.
Another option is to check out some Narnia - related forums. Sometimes, fans share their own fanfictions on these forums or recommend good ones they've found. You can interact with other fans and get more personalized suggestions for 'Prince Caspian' fanfiction.