One interesting crossover could be the characters from Total Drama facing the magical challenges in the Harry Potter world. For example, they might have to deal with magical creatures during a challenge. Another could be a character from Harry Potter ending up in the Total Drama setting and being completely out of their element, which would lead to some comical situations.
One interesting crossover could be the use of Avatar's bending abilities in the wizarding world of Harry Potter. For example, a character who can bend water could use it to create unique defenses against dark magic spells that involve fire. Another could be the idea of Hogwarts having an exchange program with the Avatar world, and students from each side learning about the other's culture and magic systems.
One interesting crossover could be the use of magic spells from Harry Potter in the Skyrim world. For example, the 'Expelliarmus' spell might be used to disarm an enemy in Skyrim instead of the normal combat disarming moves. Another could be the appearance of Hogwarts-like structures in Skyrim. Wizards from Harry Potter might also find their way into Skyrim and interact with the local mages there.
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?
One interesting crossover could be the magic system of Harry Potter combined with the ninja skills in Naruto. For example, a story where Naruto characters learn to use wands and spells while Hogwarts students learn chakra control. Another could be a plot where Voldemort teams up with Orochimaru to take over both the wizarding world and the ninja world. There could also be a story about Harry Potter being reincarnated as a ninja in the Naruto world and using his knowledge of magic in new and unexpected ways.
One interesting crossover could be the use of ninjutsu in the wizarding world. For example, Naruto's shadow clone jutsu could be used by a Hogwarts student to attend multiple classes at once, leading to all sorts of comical and chaotic situations in the hallways of Hogwarts.
One interesting crossover could be the idea of Hogwarts students having mutant abilities. For example, a student who can control the weather like Storm from X - Men. It would create a whole new dynamic within the Hogwarts environment, with new challenges and friendships.
One interesting crossover could be the magic system of Harry Potter being introduced to the MHA world. For example, a character from MHA might discover they have magical abilities like a Hogwarts student. Another could be the heroes from MHA having to face off against dark wizards from the Harry Potter universe. It would be a really cool mix of the two different power dynamics.
Another crossover could be in the form of a quest. Imagine a Reborn character joining Harry, Ron and Hermione on their search for Horcruxes. Their different skillsets would make for an interesting combination. The Reborn character might have knowledge or abilities that help the trio avoid danger more easily or even discover new Horcruxes that were previously hidden due to their unique perspective.
One interesting crossover could be the magic systems interacting. For example, how would the spells from Harry Potter work in the world of Overlord? Maybe a 'Wingardium Leviosa' spell could be used to levitate some of the items or minions in the Overlord world. Another aspect could be the characters meeting. Imagine Harry Potter and Ainz Ooal Gown having a conversation about leadership and power, since both are in positions of power in their respective stories.
A cool aspect in these crossovers might be the cultural exchange. The Mewnians from SVTFOE would be amazed by things like Quidditch. They might introduce their own magical games to Hogwarts students. Also, the Hogwarts houses could influence the relationships between the SVTFOE characters. A character like Janna might be a bit of a Slytherin, using her slyness in the new magical environment.