Luna Lovegood could be a crossover character. Her unique perspective and belief in strange magical creatures could translate well into the Vampire Diaries world. She might discover new supernatural beings in Mystic Falls that she had only read about in her own world. And then there's Klaus from the Vampire Diaries. He could be a wizard in the Harry Potter world in the fanfiction. His power - hungry and manipulative nature would make him a formidable dark wizard, causing chaos in Hogwarts with his vampire - like thirst for power.
One idea could be that the Hogwarts students are sent to Mystic Falls for a magical exchange program. Harry could meet Elena and they discover a new dark magic that is related to both the wizarding world and the vampires. Another could be that a vampire from Vampire Diaries somehow ends up at Hogwarts and has to navigate the magical rules and classes while hiding their true nature from most students. A third idea is that the Death Eaters team up with some of the bad vampires in Vampire Diaries to create chaos in both worlds.
I really like 'A Magical Merge'. In this fanfic, the Hogwarts students find themselves in Mystic Falls for some reason. It plays with the idea of how the wizards would react to vampires and werewolves. There are some great character development arcs, especially for characters like Elena who has to adapt to this new world of magic wands and spells. It also has some really cool fight scenes between the vampires and the dark wizards.
Love and friendship are also common themes. Characters from both universes form new relationships. In many fanfictions, a character from Harry Potter might fall in love with a vampire from Vampire Diaries. And the friendships that cross the boundaries of the two worlds are also explored, like how a wizard and a vampire could become friends despite their differences.
Imagine if Elena Gilbert was somehow connected to an Infinity Stone. This would draw the attention of Thanos, but also the Avengers. The Vampire Diaries characters would have to protect Elena while also dealing with the fact that the Avengers are now in their small town. There could be a lot of mistrust at first, but as they learn more about each other's motives and capabilities, they form an alliance to keep the Infinity Stone out of Thanos' hands.
I think Harry and Damon would be an interesting pair. Harry has his strong sense of justice and Damon is this complex, morally grey vampire. Their interactions would be full of tension and banter. They could learn from each other - Harry about the vampire world's darker side and Damon about the power of friendship like Harry has with his friends. Another great pair could be Hermione and Bonnie. Both are smart and powerful in their own ways. They could combine their magic knowledge to solve problems that come up in the crossover.
One common theme is the struggle between good and evil. Just like in the original works, in these fanfictions, the female characters often face dark forces. In the context of both universes, this could be Voldemort's followers in Harry Potter and the various vampires with malicious intents in Vampire Diaries.
One interesting fanfiction could be a crossover where characters from both universes meet. For example, imagine Hermione meeting Damon Salvatore. Another could be a story that delves deeper into the backstories of minor characters in both series. There are also fanfictions that rewrite certain plot points in a more creative way, like if Voldemort was somehow related to the original vampire bloodline in Vampire Diaries.
One common theme is the clash of different supernatural powers. Since Harry Potter has magic and Vampire Diaries has vampires, werewolves, and witches, the fanfiction often explores how these powers interact. Another theme is the cultural differences between the two worlds. For instance, the way Hogwarts functions compared to the social structure in Mystic Falls.
One popular trope is the 'character swap'. For example, having Harry Potter end up in the 'Vampire Diaries' world and having to deal with the vampire politics there. It's interesting to see how his Gryffindor courage plays out in a different setting. Another is the'magic - mix' trope where the magic from both universes combines in unexpected ways. Like using a spell to control a vampire or a vampire's blood having some sort of magical effect on a wizard.