One interesting crossover could be the use of magic in the Z Nation apocalyptic world. For example, a wizard from Harry Potter might use spells to fight off zombies. Maybe a Patronus charm could be used to keep the zombies at bay as it's a powerful form of positive magic.
In some fanfiction, there could be a situation where the Dark Arts from Harry Potter are somehow related to the origin of the zombie virus in Z Nation. A dark wizard might have created a spell that went wrong and led to the zombie apocalypse. The heroes from both universes would then have to team up to stop the spread of the virus and deal with the dark magic behind it. This would bring together the action - packed world of Z Nation and the magical realm of Harry Potter in a really exciting way.
One interesting crossover could be the vampires from Twilight having to deal with the zombie apocalypse in Z Nation. Maybe they use their superhuman abilities to fight off the zombies in unique ways.
One interesting 'Harry Potter Z Nation' fanfiction could be one where the characters from Harry Potter have to face the zombie apocalypse situation from Z Nation. For example, Harry and his friends might use magic to fight off the zombies. Maybe Hermione would try to find a magical cure for the zombie virus using her vast knowledge of spells and potions.
Some crossovers might involve the Dragon Balls being used in the Harry Potter universe. Maybe the wizards would be very interested in the power of the Dragon Balls to grant wishes. For example, Dumbledore could be intrigued by their potential and try to study them. Or Harry Potter himself could accidentally find a Dragon Ball and have to figure out what it is while also dealing with Voldemort. It would be a wild mix of two very different magical worlds.
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 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 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.