One interesting crossover could be Stewie from Family Guy getting his hands on a Harry Potter wand and causing chaos at Hogwarts. He might use his advanced intellect to try and outwit the wizards and witches there.
Imagine Pugsley Addams getting a Hogwarts letter. He'd be so excited to learn magic, especially dark magic. He could get into trouble by trying to create his own spells inspired by the Addams family's unique interests. Meanwhile, the Addams family's butler, Lurch, could be a half - giant like Hagrid. He could interact with Hagrid and they could share stories about taking care of strange creatures.
In some fanfictions, there could be a plot where the dark magic in the Harry Potter universe intersects with the Addams family's natural inclination towards the macabre. Maybe a dark wizard tries to use the Addams family's home as a base for some evil plan. But the Addams family, with their own brand of strength and eccentricity, turn the tables on the wizard. This could lead to an alliance between the Addams family and the Hogwarts faculty to fight off the common threat.
A more complex plot could be that the entire Family Guy cast gets transported to Hogwarts during a school year. Quagmire could be the new Defense Against the Dark Arts teacher, bringing his own unique brand of 'defense' which mostly involves flirting with the female students. Lois could get into trouble for trying to organize a Muggle - style PTA at Hogwarts, and Peter could end up on the Quidditch team as an accidental addition.
One interesting crossover could be the Addams family's spooky mansion being a hidden location in the wizarding world. Maybe the ghosts in the Addams family mansion are actually the result of some botched wizarding spell long ago.
One interesting crossover could be the Griffins finding themselves in the world of RWBY. Maybe Stewie would be intrigued by the unique weapons and semblances. He might try to figure out how to incorporate some of those elements into his own evil plans.
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.