Another possibility is a character from the Harry Potter world being related to Poseidon in some way. For instance, a pure - blood wizard family might have an ancient connection to Poseidon. This connection could bring about unique storylines, like the family having special powers related to the sea that they have kept hidden for generations and now, with the help of some events in the Harry Potter story, these powers start to surface.
One interesting crossover could be a magical duel in Hogwarts where Poseidon's powers are somehow involved. Maybe a character with Poseidon - like abilities challenges a powerful wizard from Harry Potter world.
In some 'Harry Potter and Poseidon fanfiction', there might be a story where the Hogwarts students go on an underwater adventure and encounter Poseidon's realm. They could learn about different kinds of magic there that are related to the sea. For example, the mermaids in the Harry Potter series could have a deeper connection to Poseidon's power and they help the students understand the underwater magic better.
One interesting crossover could be Ben using his alien forms to help the students at Hogwarts during a dark wizard attack. For example, his Heatblast form could be useful against ice spells. Another could be Harry Potter using magic to enhance Ben's Omnitrix. Maybe a spell that makes it transform faster or unlock new alien forms.
One interesting crossover could be the use of magic from Harry Potter in the RWBY universe. For example, a character from RWBY might learn how to use spells like Wingardium Leviosa to help in combat or daily tasks. Another could be the Hogwarts houses being applied to RWBY characters, with Ruby perhaps being a Gryffindor due to her bravery.
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.
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.
One interesting crossover could be the use of magic systems. For example, how would the Shadowhunter runes interact with Harry Potter's spells? Maybe a character from the Shadowhunter world could find themselves at Hogwarts and be both amazed and confused by the different forms of magic. Another aspect could be the characters' personalities clashing or working together. A brave Gryffindor and a fearless Shadowhunter might team up to fight a common evil that threatens both their worlds.
One interesting aspect could be the contrast in magic systems. In Harry Potter, magic is more about spells and potions, while Starscream has his own set of technological 'powers' in the Transformers universe. A crossover might explore how they interact. For example, how would Starscream react to a wizard's ability to apparate? And how would a wizard deal with Starscream's flight and weaponry capabilities.
In Harry Potter and The Flash fanfiction, a common theme is the exploration of different forms of transportation. The Flash's super - speed is a form of getting around quickly, while in Harry Potter, they have brooms, Portkeys, and the Floo Network. A fanfiction could explore how the Flash would react to these magical means of travel. Maybe he'd be impressed by the idea of flying on a broom or find the Floo Network's green flames a bit strange. And on the other side, Harry Potter characters could be amazed at how the Flash can run across water or up the sides of buildings just by using his speed.
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.
In certain Harry Potter and Aladdin fanfictions, there are crossovers where the magic of Aladdin's world and the magic of the Harry Potter universe interact in unique ways. For example, the jinni from the lamp could teach Defense Against the Dark Arts for a semester at Hogwarts. The jinni would bring in a whole new set of spells and techniques from the Arabian magical traditions. And perhaps Harry Potter himself could be transported to Agrabah and have to use his magic skills to solve the problems there, like dealing with Jafar using only his wizarding skills instead of relying on the lamp.
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.