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.
There are numerous fanfictions that are based on the concept of time - travel. For instance, a character from Harry Potter might travel back in time to the era of the first vampires in Vampire Diaries. This could lead to all sorts of interesting interactions and changes in the timelines of both universes. Another type of fanfiction could be about the characters' moral struggles. Picture Harry Potter facing the same moral dilemmas as the vampires in Vampire Diaries when it comes to taking human lives for survival. There are also fanfictions that simply focus on the friendship between characters from the two series, like how Harry could befriend Elena and help her deal with the supernatural chaos in her town.
One interesting fanfiction could be about a wizard from 'Harry Potter' ending up in the world of 'The Vampire Diaries' and having to navigate the different magical rules and vampire politics. Maybe they form an alliance with the Salvatore brothers to fight against a common dark magic threat.
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.
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.
Well, it's the combination of two very different and popular universes. Harry Potter has its magic schools and spells, while Vampire Diaries has vampires, werewolves and witches in a different context. When they are mashed together in fanfiction, it creates a whole new world of possibilities for stories.
Another interesting aspect is how the different worlds' rules are bent or combined. For instance, the concept of a Horcrux in 'Harry Potter' and the way vampires can be killed in 'Vampire Diaries'. In some fanfictions, these two ideas are merged. Maybe a vampire tries to use a Horcrux - like mechanism to avoid being killed, or a wizard has to figure out how to destroy a vampire in a way similar to destroying a Horcrux. This creates a whole new set of challenges and mysteries for the characters to solve.
One interesting Vampire Diaries fanfiction could be about an alternate universe where Elena is a vampire from the start. It changes the whole dynamic of her relationships. Another might focus on Damon and Bonnie's friendship, exploring how they could team up in different ways than in the show.
A really cool Vampire Diaries fanfiction story could be one where the Salvatores and the Mikaelsons team up against a greater evil. This would be a great way to see different characters with their own motives working together. There are also fanfictions that focus on the human side of things more. For instance, a story about Matt's life and how he copes with all the supernatural chaos around him, and how he might even find a way to fight back using his own human strengths.
One interesting story could be about Jeremy exploring his new vampire powers in a way that the show didn't fully explore. For example, he might use his vampire abilities to protect the town from a new threat that's not related to the typical vampires or werewolves. Maybe he teams up with some unexpected characters to fight this new danger.
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.