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 idea could be that Peter gets lost in the forest of Narnia on his own during a quest and has to rely on his wits and some unexpected help from a minor character in the Prince Caspian story to find his way back to the main group. Another could be a time - travel twist where Peter is sent back to a crucial moment in Prince Caspian's past and has to figure out how to change things without disrupting the future of Narnia too much. And perhaps a story where Peter and Prince Caspian have a misunderstanding that leads to a mini - adventure to resolve their differences and strengthen their bond.
Some fanfictions also pair Lucy with Caspian. Lucy has her own unique magic and kindness, and when paired with Caspian, it can create interesting relationships, like her helping Caspian understand the more magical aspects of Narnia.
One idea could be a story where Peter and Caspian have to go on a secret mission together to find a lost magical artifact that can save Narnia from a new threat. They start off not getting along but gradually learn to respect each other's skills.
Some fanfictions focus on the relationship between Peter and Caspian when they first meet. It could explore how they initially view each other with suspicion, being from different worlds in a way. But as they start to understand each other's motives and the importance of their roles in Narnia, they form a strong bond that helps them protect Narnia from various threats.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Peter and Caspian. Just search for their names in the search bar, and you'll get a list of relevant stories.
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 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.
Peter could be developed further by having him face a personal fear. Maybe he has a fear of losing his connection to Narnia completely. Caspian, on the other hand, could be developed by exploring his relationship with his people on a deeper level. He could be shown dealing with the everyday problems of his subjects, which makes him more relatable and also shows his growth as a king. Their characters could also be developed through their interactions with each other. They could have friendly competitions that teach them both about their own strengths and weaknesses.
A great plot could be a time - travel related one. Maybe something goes wrong in Narnia, and Peter and Caspian are sent back in time to the very beginning of Narnia's creation. They have to figure out how to get back to their own time while also making sure that the events that led to Narnia's creation still happen as they should. They meet all kinds of ancient beings and learn secrets about Narnia that no one else knows.