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.
A great crossover concept in fanfiction is the idea of the Marauder's Map being used in Narnia. Imagine if Peter, Susan, Edmund, and Lucy could use the map to find their way around Narnia, or if the map could show them the location of the White Witch. And on the other hand, the magical items from Narnia like the Wardrobe could be used in the Harry Potter universe. Maybe it could be a portal to a hidden part of Hogwarts or a place where dark magic is stored, and the characters have to go on an adventure to deal with it.
One interesting crossover could be the characters from Hogwarts finding a magical portal to Narnia. They might be amazed by the talking animals and the different magical rules there. For example, Harry could use his magic skills to help the Narnians in a battle against the White Witch.
A really cool concept in this type of fanfiction is when the magic items from both universes interact. Like, what if the Philosopher's Stone was brought to Narnia? How would it affect the balance of magic in Narnia? And how would the Narnians react to such a powerful and mysterious object?
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 some fanfiction, we might see Anna and Kristoff visiting Hogwarts. They could be amazed by the moving staircases and magical creatures. In reverse, Harry, Ron and Hermione could end up in Arendelle. Hermione might be intrigued by the way Elsa controls her ice powers and try to find a scientific or magical explanation for it. Also, the Dementors from Harry Potter could be compared to the dark forces in Frozen 2, like the mist that Elsa has to face.
One interesting crossover could be the use of magic in the Avengers world. For example, Hermione could use her knowledge of spells to help the Avengers solve a problem that technology couldn't. Maybe she could use a spell to break through an impenetrable force field created by an enemy. Another aspect could be the Avengers characters learning magic. Tony Stark might be fascinated by the concept of magic and try to incorporate it into his technology. Or Thor could share his knowledge of Asgardian magic with the Hogwarts students.
One interesting crossover could be a scenario where Hermione uses her magic to help Elsa control her powers better. Maybe she creates a spell that can soothe Elsa when she's about to lose control.
In some fanfiction, Maleficent might team up with a dark wizard from the Harry Potter universe. They could combine their powers to try and take over both the wizarding world and the fairy - tale land where Maleficent comes from. This would create a complex power struggle with the heroes having to find new ways to combat them.
One interesting crossover could be the use of spells. For example, a Charmed one might learn a spell from the Harry Potter world like the Patronus charm. It could be used in a new and different way in the Charmed universe, perhaps to fight a new type of demon that is vulnerable to positive magic like a Patronus.
One interesting crossover could be Superman visiting Hogwarts. He might be amazed by the magic there. Maybe he tries to learn some spells, but his superpowers interfere. For example, when he tries to use a levitation charm, his super strength makes the object fly out of control.
One interesting crossover could be a scenario where Harry Potter and his friends encounter Janemba's chaotic powers in the wizarding world. Maybe Janemba's otherworldly abilities disrupt the magic there, and Harry has to use his knowledge of both magic and the strange new power he's facing to set things right. For example, he might combine spells with his natural courage to combat Janemba's minions.