One interesting plot twist is the discovery that Elena has a doppelganger. It adds a whole new layer of mystery and danger as different characters have different motives regarding her look - alikes. Another is when Damon turns into a human for a while, which completely changes his dynamic with other characters.
One interesting plot twist could be that the father of Caroline's baby is a long - lost relative of an original vampire. This would bring in a whole new level of drama as different vampire factions might try to either protect or harm Caroline and her unborn child depending on their own motives.
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.
A really interesting plot twist is when a character from Arrow gets turned into a vampire in some fanfictions. For example, in 'The Dark Transformation', Felicity Smoak gets bitten by a vampire and has to deal with her newfound vampiric nature while still trying to help Team Arrow. It completely changes her character dynamics and adds a whole new layer of complexity to the story.
In some fanfics, a plot twist could be that a vampire from Vampire Diaries turns out to be a long - lost relative of a Hogwarts student. This completely changes the dynamics as the student has to deal with their newfound family connection and the fact that they're a vampire.
One interesting plot twist could be that the vampire hunters turn out to be vampires themselves in disguise. This completely flips the expected dynamic and adds a whole new layer of mystery and danger.
One idea could be to cross it with another supernatural show like 'Teen Wolf'. The characters from both could team up to fight a common enemy, like an ancient and powerful vampire - werewolf hybrid. For example, Elena could use her charm and vampire skills, while Scott McCall could lead his pack in the fight.
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 popular plot twist could be that Elena discovers she has a hidden power related to her Petrova heritage that was previously unknown. For example, she might be able to control other vampires' minds briefly. Another twist could be that Katherine isn't as bad as she seems and has been secretly protecting Elena all along. Maybe she made a deal with an ancient vampire to keep Elena safe in exchange for something. A third plot twist could be that there is a third Petrova sister that no one knew about, and she emerges with powers that are a combination of Katherine and Elena's, causing chaos in Mystic Falls.
One interesting plot twist could be that Buffy becomes a vampire but still retains some of her Slayer powers. This creates an internal struggle as she has to fight her new vampire instincts while also using her old skills. Another could be that she turns into a vampire and finds out that there's a whole new level of vampire society that she was unaware of, which changes her perspective on the whole vampire - slayer dynamic.