Well, a cool crossover might be having Frankenstein's monster end up in the wizarding world. Picture the monster being found by Hagrid. Hagrid, being Hagrid, would probably feel sorry for the monster and try to help him fit in. The monster could then befriend some of the Hogwarts students, like Neville. Neville could teach the monster about plants, and in return, the monster could protect Neville from bullies. Or there could be a plot where Voldemort tries to use the monster for his own evil purposes, thinking he can control it, but the monster has a sense of right and wrong and rebels against Voldemort.
Some crossovers could involve the magic in the Harry Potter universe being used to 'fix' Frankenstein's monster. For example, a powerful wizard or witch could use a spell to give the monster a more normal appearance or to endow it with the ability to speak more clearly. In reverse, Frankenstein's scientific methods could be introduced to the wizarding world. Maybe a Muggle - born wizard who was a big fan of Frankenstein before discovering their magic tries to combine magic and science. This could lead to new and unorthodox forms of magic that are initially met with suspicion but later prove useful in the fight against dark forces.
One idea could be to combine the magic of Harry Potter with the concept of creating life in Frankenstein. For example, a Hogwarts student could attempt to use magic to bring a creature to life in a similar way to Frankenstein's experiment, but with magical twists. Maybe they use a spell on a patchwork of magical creatures' parts. Another idea is to have a character from Frankenstein's world end up at Hogwarts and deal with the magical environment and its rules, while also bringing their own knowledge of creating life.
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.
In Harry Potter and Clannad fanfiction, a great crossover idea is the exchange of character traits. Maybe some of the Clannad characters could be sorted into Hogwarts houses based on their personalities. And vice versa, the Hogwarts students could learn about the strong family and friendship bonds in Clannad. It could also involve a plot where a powerful Clannad character has to deal with the dark magic threats in the Harry Potter universe. This would create a whole new set of challenges and adventures for both sets of characters.
In some Harry Potter and Riverdale fanfiction, the houses in Hogwarts could be compared to the social groups in Riverdale. For example, Gryffindor's bravery could be related to how the Bulldogs in Riverdale stand up for what they believe in. Slytherin's cunning might be similar to how some of the more devious characters in Riverdale operate. And then there could be a plot where characters from Riverdale are sorted into Hogwarts houses and have to adapt to the new environment based on the traits of their houses.