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.
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.
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.
A group of powerful hunters who have turned to the dark side could also be antagonists. They see Artemis as a threat to their new - found power and also target Peter Parker because of his association with her. They could have advanced weaponry and hunting skills that rival Artemis', and they would constantly be on the prowl to take down our heroes.
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.
A new character could be created as the main antagonist. For example, a powerful sorcerer from another dimension who wants to steal the powers of both Peter and Valkyrie. This villain could be after the spider - powers and Valkyrie's Asgardian strength to increase his own power and take over the MCU universe.
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 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.
It could be Voldemort who somehow gets his hands on Oscorp technology and uses it to make his dark magic even more powerful. He would be a formidable foe for both Harry and Peter, as he has his own army of Death Eaters and now enhanced powers. Harry would use his magic and Peter his superpowers to fight against Voldemort's new - found strength.
It could be Hydra. They might be trying to take advantage of the split between the Avengers and the Rogue Avengers. They see Peter as a wild card and try to either capture or turn him against both groups.
It could be Voldemort. Since he's the main villain in the Harry Potter series, he'd see Peter as another threat to his power, especially if Peter starts interfering in the wizarding world with his newfound family in Harry.