Another possibility is that Voldemort somehow discovers the world of Narnia and tries to conquer it. This would force the Narnian characters to team up with the Order of the Phoenix. Harry Potter and his friends could learn about the talking animals and different forms of magic in Narnia, while the Narnian heroes would see the power of wizardry. It could lead to a great adventure full of battles and new friendships.
Well, in Narnia, there's the magic of Aslan which is more of a divine and nature - based magic. In Harry Potter, it's more about spells and potions. You could have a situation where a wizard tries to learn the magic of Aslan. For example, Hermione might be intrigued by the magic that brings Narnia to life and tries to study it. She could combine her knowledge of wizardry with the Narnian magic to create new spells.
One idea could be that the Golden Trio from Harry Potter somehow stumbles into Narnia. Maybe they're on the run from Death Eaters and a magic portal opens up, leading them to Narnia. There, they could meet the Pevensie children and together, they use their combined magical knowledge and skills to fight against an evil force that threatens both worlds.
Imagine if the Marauder's Map could show the layout of Narnia. Harry and his friends could use it to navigate through Narnia, which would be a huge advantage. But then, there could be a twist where the map starts to malfunction in Narnia because the magic there is different. And a character like Mr. Tumnus could be revealed to have some connection to the wizarding world in Harry Potter. Maybe he was once a wizard who was exiled to Narnia and lost his magic, but regains it when the two worlds start to interact.
Harry Potter and Lucy Pevensie would make an interesting pairing. Harry is used to being the hero in his own world, but Lucy has a unique kind of innocence and faith in Narnia. They could learn a great deal from each other. Harry could teach Lucy about more complex forms of magic, while Lucy could show Harry the power of believing without seeing, which is a big part of her relationship with Aslan.
One common theme is the exploration of different magical systems. Since both Narnia and Harry Potter have their own unique forms of magic, fanfictions often compare and contrast them. For instance, the magic in Narnia is more closely tied to the will of Aslan and the natural order of the world, while in Harry Potter it's more about spells and potions.
Perhaps the Philosopher's Stone somehow gets transported to Narnia. Harry and his friends follow it there. In Narnia, they find that the stone has a different kind of magic which is intertwined with the magic of Narnia. They have to learn about Narnia's magic system while also protecting the stone from the villains in both worlds.
The wizards from Harry Potter would be in awe of Narnia's magical creatures. Harry, for instance, would be fascinated by Aslan. He'd probably see Aslan as a powerful and wise being, much like Dumbledore. On the other hand, the Narnians might be a bit scared of some of the dark magic in the Harry Potter world. For example, the Dementors would be a terrifying sight for them.
One interesting crossover could be the characters from Harry Potter visiting Narnia. For example, Harry, Ron, and Hermione might find themselves in Narnia through some magical accident. They could then interact with the talking animals and magical beings there, like how they interact with the magical creatures in their own wizarding world. Maybe they would help the Narnians in a battle, using their unique Hogwarts - taught magic.
One unique feature is the combination of different magical systems. The magic in Harry Potter is more about spells and potions, while Narnia has its own brand of magic like the power of Aslan. In the fanfiction, these two types of magic interact in new and exciting ways.