One idea could be a crossover where a character from Twilight finds themselves in the Vampire Knight world. For example, Bella could end up at Cross Academy. Maybe she'd be mistaken for a new student and have to navigate the complex social structure there, with the Day and Night classes. Another plot could involve a vampire from Vampire Knight traveling to Forks. They'd have to deal with the Cullens and the werewolves, which would create a whole new set of power dynamics.
A possible plot is that the Volturi from Twilight somehow get wind of the pureblood vampires in Vampire Knight. They might see them as a threat or as something to be studied. This could lead to a conflict between the two groups of powerful vampires. Or, a fanfic could focus on the love interests in both series. For instance, a love triangle between a Vampire Knight character, a Twilight character, and someone from their own respective worlds.
One idea could be a story where a new student arrives at Cross Academy who has a unique power that disrupts the balance between the vampires and the humans. This new character could be a hybrid of some sort, and their presence forces the main characters like Yuki and Zero to re - evaluate their beliefs and relationships.
Well, an interesting plot could be that the OC is a transfer student who is secretly a vampire hunter but falls in love with a vampire at the academy. It would create a great internal conflict for the character. Also, the OC could be a half - vampire, half - human who is trying to find her place in both worlds. Maybe she has an ability that neither vampires nor humans have, like the power to heal vampire bites on humans. This would make her a target for both sides who want to use her power for their own purposes.
A more action - packed plot could be the red knight leading a rebellion against the vampire hierarchy. They gather other vampires and humans who are tired of the oppression and fight for a new order. Along the way, they discover secrets about the vampire world that could change everything.
A possible plot is that a dark force in both the Inuyasha universe (connected to Kagome) and the Vampire Knight world is trying to take over. Kagome and the main characters from Vampire Knight, like Zero and Kaname, have to work together. Kagome's knowledge of magic and spiritual energy could be combined with the vampires' powers to combat this threat. They might start off distrusting each other but gradually learn to rely on one another.
Well, a plot could revolve around Kanme's struggle with his vampire nature in a different way. For example, he discovers a new power within him that he doesn't understand and has to learn to control it while also dealing with the politics and power struggles in the vampire world. Also, a story where Kanme forms an unlikely alliance with a group of humans who are trying to understand vampires better could be really engaging.
Well, a plot could center around a time - travel aspect. Imagine if a character from the present in Vampire Knight Zero accidentally travels back to a crucial point in the vampire - human history. They could then change some events that have a ripple effect on the current story. Or, a fanfiction could be about a secret society within the vampire world that has been hidden from the main characters. This society could have its own rules and goals that conflict with what the protagonists believe. Additionally, a story about a curse that affects not only vampires but also humans in a unique way could be really engaging.
Maybe there's a Halloween prank gone wrong. A group of human students decides to play a spooky prank on the vampires, but they accidentally unleash an old vampire curse that starts to affect the whole academy. The characters then have to find a way to break the curse before it's too late.
One idea could be a crossover where characters from Vampire Academy and Twilight meet. For example, Rose Hathaway from Vampire Academy could encounter Bella Swan from Twilight. They could compare their different experiences with vampires and dhampirs. Another idea might be a 'what if' scenario. What if Lissa Dragomir from Vampire Academy was turned into a vampire instead of remaining a Moroi? In Twilight fanfiction, a cool idea could be exploring the backstory of the Volturi in more detail, like how they came to be so powerful.
One idea could be a story where a vampire - hybrid has to choose between the vampire and human worlds. It's a struggle as both have things they love. Maybe they have a human family they care about but also the allure of the vampire community with its power and immortality.
One idea could be having Elena from 'The Vampire Diaries' meet Bella from 'Twilight'. Maybe they discover that they have more in common than they thought, like their love for vampires who are trying to resist their more primal instincts. They could team up to help their vampire boyfriends deal with some common threats in both their worlds.