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 plot idea could be a female Harry Potter character finding herself in the Vampire Diaries' Mystic Falls. She could use her magic to deal with the vampire and werewolf politics there. For example, she might use a spell to protect herself from a vampire's compulsion. Another idea is that she teams up with Elena and the gang to fight a common enemy, like a powerful dark witch who has ties to both the wizarding world and the supernatural world of Mystic Falls. And she could introduce some unique magical items from the Harry Potter universe to help in their battles.
A common theme is the clash of two different fictional universes. Since Harry comes from the wizarding world and the Vampire Diaries has its own set of rules regarding vampires, witches, and werewolves, seeing how these two worlds interact is often explored.
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.
One great fem Harry Potter vampire fanfiction is 'The Vampire's Apprentice'. It takes the idea of Harry being a girl and adds the vampire element in an interesting way, with a lot of action and a unique take on the magical world.
I also like 'Fem Harry: A Magical Twist in the Vampire Diaries'. Here, fem Harry brings a whole new dimension to the Vampire Diaries story. Her unique magic causes quite a stir among the vampires and witches. The author does a great job of blending the two universes together, and the character development of fem Harry as she navigates this new world is really well - written.
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.