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.
One more cool plot twist can be seen in 'Unexpected Bonds'. Here, Damon Salvatore forms an unexpected bond with Malcolm Merlyn. They start off as enemies but due to some supernatural events, they end up having to rely on each other. This changes the power dynamics between the good and bad guys in both shows and makes for a very engaging read as you wonder how their relationship will develop and how it will affect the overall story.
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.
The clash of powers. Naruto's chakra - based jutsu against the vampires' superhuman abilities like speed and compulsion. It makes for some exciting battle scenes.
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 a character from Rosario Vampire getting transported to the Ranma universe and having to adapt to its unique martial arts and gender - changing situations. For example, Moka could end up in Furinkan High School and have to deal with Ranma's crazy antics. Another plot twist might be a fusion of the two worlds' magic systems. Maybe the nekos from Rosario Vampire learn some of Ranma's martial arts techniques and combine them with their own powers to create new and powerful attacks. A third plot twist could involve a love triangle between a character from each series and a new character created in the crossover. This would add a whole new layer of drama and conflict to the story.
One interesting aspect is the character mash - ups. Seeing how characters like Edward from 'Twilight' would interact with Elena from 'The Vampire Diaries' can be really exciting. Their different personalities and backgrounds create a lot of potential for interesting dialogues and relationships.
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.
One possible plot could be that the Avengers are investigating a series of strange disappearances in a small town. They soon discover that vampires from The Vampire Diaries' world are behind it. There could be a face - off between Thor's hammer and the super - strength of some of the older vampires. Maybe Damon could team up with Iron Man, with their shared sarcastic sense of humor, to outwit the main villain who is causing chaos in both their worlds.
One interesting plot idea could be a battle between the hunters from Supernatural and the vampires from Vampire Diaries. Maybe the Winchesters end up in Mystic Falls and think all the vampires there need to be hunted. Another idea is a character swap. For example, Castiel could end up in the Vampire Diaries world and interact with the characters there in a very different way compared to his usual interactions in Supernatural. A third idea could be a joint mission to fight a greater evil that threatens both universes, like an ancient demon that wants to use the power of the vampires in Vampire Diaries and the magic from Supernatural for its own dark purposes.
A really unique plot twist in DC Vampire fanfiction can be that the Green Lantern Corps has to deal with a vampire - like entity that feeds on the energy of their rings. This forces them to find new ways to power their rings and fight this entity. And sometimes, a plot twist is that a vampire version of Aquaman rules the underwater world, and the normal Aquaman has to figure out how to stop him without getting turned into a vampire himself.