Some fanfictions might focus on the idea of the Shield Hero attending Hogwarts. He could be sorted into a house, perhaps Hufflepuff for his loyalty. Then he would have to learn magic while also dealing with the threats he's used to in his own world. The magic spells could be used to modify his shield or give him new abilities. Also, the villains from 'Harry Potter' could team up with the villains from 'Shield Hero', creating a really complex and challenging situation for the heroes to overcome.
One interesting crossover could be the use of magic from 'Harry Potter' in the world of 'Shield Hero'. For example, the Shield Hero might encounter a wizard who helps him with a spell to enhance his shield. Another could be the characters from 'Harry Potter' having to face the same kind of prejudice that the Shield Hero does in his own world. Maybe Hermione could use her intelligence to come up with strategies similar to what the Shield Hero uses in battles.
A cool crossover in the fanfiction could be that some of the spells from Harry Potter are used in the Big Hero 6 world. Like the invisibility cloak could be something that the Big Hero 6 team tries to replicate using their technology. Or the characters from Big Hero 6 could help the Harry Potter crew in a battle using their advanced robotics and inventions.
One interesting crossover could be the characters from Naruto learning about the unique shield abilities in Rising of the Shield Hero. For example, Naruto might be intrigued by Naofumi's shield and how it can transform and be used in different ways. Another aspect could be the ninjas from Naruto teaching the heroes in Shield Hero some of their combat techniques.
One interesting crossover could be the characters from My Hero Academia attending Hogwarts. For example, Deku might be sorted into Gryffindor due to his courage. He could learn magic alongside spells like Wingardium Leviosa while still having his Quirk. Another could be Bakugo being a Slytherin and causing chaos with his explosive temper in the magical school. And All Might could be like a powerful wizard figure, perhaps even a guest lecturer at Hogwarts.
There could also be a scenario where Harry Potter has to face the Joker in a psychological battle. Since Harry has faced his own share of dark forces, the Joker's mind - games could be a new and different kind of challenge for him. Harry would have to use his courage and the skills he learned at Hogwarts to outwit the Joker.
One interesting crossover could be the use of magic in the Warframe universe. For example, a Harry Potter character might bring spells like 'Accio' to retrieve items in the middle of a Warframe mission. Or perhaps a Warframe with magical abilities inspired by Hogwarts magic.
One interesting crossover could be a Creepypasta character infiltrating Hogwarts. For example, Jeff the Killer could be lurking in the Forbidden Forest, adding a terrifying element to the otherwise magical place. It would be a really spooky scenario as students unknowingly get closer to this dangerous figure during their adventures in the forest.
One interesting crossover could be a scenario where Alucard uses his vampiric powers to assist Harry in a battle against Voldemort. Maybe Alucard's super strength and ability to transform could give Harry an edge in a difficult fight.
Well, a cool crossover might involve the Grimm from RWBY being affected by the magic in Harry Potter. Maybe the Death Eaters could team up with some of the more evil forces in RWBY. Also, the different houses in Hogwarts could be compared to the teams in RWBY. For instance, Gryffindor's courage could be related to Team RWBY's fearless nature.
A great crossover idea in the fanfiction could be a Quidditch match between Hogwarts and Alfea. The flying skills of the Winx fairies would add a new dimension to the game. And the strategic play of the Hogwarts students would also be interesting to see in combination. Also, the different brooms and flying devices could be compared and contrasted.
A possible crossover could involve the dark forces in both universes. Voldemort and the forces of evil in Harry Potter could team up with some of the villains from Asgard. This would create a huge conflict where the heroes from both worlds, like Harry Potter and the Avengers - like heroes of Asgard, would have to combine their powers to defeat the combined evil.