Maybe a demigod gets turned into a vampire in a strange magical accident. This would create a whole new set of challenges as they try to balance their demigod powers with their new vampire nature. They might have to deal with the vampire politics in Mystic Falls while also trying to stay true to their Olympian heritage. It could lead to some really interesting character development and power struggles.
Another interesting plot could involve a vampire coven that has some sort of connection to the Greek gods. Maybe they want Percy for his unique demigod blood, which they believe has special powers. So Percy has to fight against this coven to keep his blood and his friends safe. This would bring in elements of both the Percy Jackson world and vampire lore. And it could lead to some really cool battles and alliances.
In some fanfics, a plot twist could be that a demigod turns into a vampire. This completely changes the dynamic as they now have to deal with their new vampiric nature while still having their demigod powers. It's a really unique take on the crossover.
Some fanfictions might explore how the demigods' weapons work against vampires. For instance, Percy's Riptide could potentially be very effective against vampires, as it is a magical weapon. And the vampires' speed and strength could pose a real challenge to the demigods. It would be fascinating to see how the characters from both series adapt to each other's worlds. Maybe the demigods would have to learn about the rules of the vampire world, like not being able to enter a home without an invitation, while the vampires would be shocked by the existence of gods and demigods.
One idea could be being reborn as a long - lost relative of the Salvatore brothers. Maybe you have some hidden power that was lost in the family history and now you have to navigate the vampire - filled world of Mystic Falls while trying to figure out your new abilities. Another plot could involve being reborn as a witch in the fanfiction world. You could use your magic to either help or hinder the vampires and other supernatural beings in town. You might also be reborn as a human with a special gift of seeing the future, which would put you in a dangerous position as everyone in the Vampire Diaries world would want to use your power for their own ends.
One idea could be being reborn as a long - lost relative of one of the main characters. For example, a long - lost sister of Elena. This could stir up a whole new dynamic in the Mystic Falls gang. You could bring in new powers or a different perspective on the vampire - werewolf - witch drama.
One idea could be a story where Elena discovers a long - lost relative who is also a powerful vampire hunter. This relative tries to turn Elena against her vampire friends, creating a lot of internal conflict for her.
One idea could be a powerful vampire using mind control to make Elena forget her love for Stefan and fall for him instead. This would create a lot of drama as Stefan tries to figure out what's wrong and win her back.
One idea could be waking up as a new vampire in Mystic Falls. You could explore the confusion and newfound powers. Another could be waking up in the middle of a vampire - hunter conflict, having to quickly choose a side. Maybe you wake up with a strange connection to one of the main characters, like having their memories or a special bond with Damon or Stefan.
One interesting plot idea could be Selena Gomez as a powerful siren who lures vampires in Mystic Falls. Her character would have the ability to control them with her voice, creating a new threat to the vampires and other supernatural beings in the town.
One plot idea could be a black cat that is actually a shapeshifter. In the world of Vampire Diaries, it could be a witch who was cursed to take the form of a cat. The cat could befriend Elena and help her in her battles against the vampires. Maybe it has some special powers like being able to sense danger from afar. Another idea is that the black cat is a familiar to an ancient vampire. This vampire uses the cat to spy on the other characters. The cat could also be a key to unlocking a hidden mystery in Mystic Falls, like a secret passage or a long - forgotten spell.