There are several places. AO3, as mentioned before, is excellent. It has a wide range of stories, from the popular crossovers to more niche ones. DeviantArt can also be a source. Sometimes artists also write fanfic, and you might find some Vampire Knight crossover fanfic there. Additionally, some independent fanfic websites might have what you're looking for. However, be cautious when using these smaller sites as they may not have the same level of moderation as the bigger platforms.
Well, AO3 is definitely a top choice. It has a tagging system that makes it easy to find Vampire Knight crossover fanfic. Just search for 'Vampire Knight' and then add the name of the other fandom you want it to be crossed with. Wattpad also has some Vampire Knight crossover fanfic. You might have to dig a bit more through the search results, but it's worth a look. And don't forget about Reddit. There are some subreddits dedicated to fanfiction where you might find Vampire Knight crossover fanfic or get recommendations from other fans.
A great Vampire Knight crossover fanfic could involve a crossover with 'Naruto'. The ninja skills and the chakra system in Naruto could be pitted against the vampire powers in Vampire Knight. It could be a battle of different power systems. Another possibility is a crossover with 'Attack on Titan'. The Titans could pose a new kind of threat to the vampires, and the vampires might have their own ways of dealing with the Titans. Also, a crossover with 'Sailor Moon' could be fun. The Sailor Scouts' magic and the vampires' powers could clash or combine in unexpected ways.
Power is a big theme. The powers of the Sailor Scouts, which are based on elements like the moon and stars, are different from the vampiric powers in Vampire Knight. In crossover fics, there's often a comparison or a combination of these powers. Also, the idea of good vs evil is common. Sailor Moon and her friends are the defenders of love and justice, while vampires in Vampire Knight are often seen as more morally ambiguous. So seeing how they interact in the face of threats is a theme. Sometimes, there's also the theme of transformation. Both series have elements of transformation, and in crossovers, it can be interesting to see how the two types of transformations interact.
A possible plotline could be that the Ghost Rider's power somehow leaks into the Vampire Knight world. For example, a strange portal opens and the essence of the Ghost Rider enters Cross Academy. This causes chaos as the vampires and vampire hunters are affected in different ways. Some vampires might be weakened by the holy fire aspect of the Ghost Rider, while some hunters might gain new powers. The characters then have to figure out how to close the portal and restore balance.
One great Vampire Knight and Ghost Rider crossover fanfic might be titled 'The Night Riders of Cross Academy'. In this fic, the dark and brooding world of Vampire Knight meets the hellish and vengeful world of Ghost Rider. The main character, perhaps Zero, could gain the powers of Ghost Rider, adding a whole new level of badassery to his fight against the vampires at Cross Academy. It could explore how his new powers affect his relationships with Yuki and the other characters.
One great Vampire Knight Sailor Moon crossover fanfic is 'Moonlit Knights'. It combines the dark and brooding atmosphere of Vampire Knight with the magical girl elements of Sailor Moon in a really interesting way. The characters from both universes interact in unexpected ways, like how Usagi might react to the vampires in Vampire Knight. Another one could be 'Starlit Bloodlines' which delves deep into the lore of both series and creates a unique story line where the Sailor Scouts have to deal with the vampire world.
Edward from Twilight and Kaname from Vampire Knight could make an interesting pair. Edward has his brooding and protective nature, while Kaname is a powerful pureblood with his own secrets. Their encounter could be full of tension as they both have strong personalities and different views on being a vampire. Maybe they'd start off as rivals but then find common ground in protecting their loved ones.
Another theme is power. Naruto is known for his strong powers in his own universe, and in the Vampire Knight world, there are different power dynamics among the vampires. Fanfictions may show how Naruto's powers compare or combine with those of the vampires. For example, his chakra could have unexpected effects on vampire abilities. It's really interesting to see how the power balance is adjusted in these crossovers.
Love and forbidden relationships are also very common. Since werewolves and vampires are often depicted as enemies in their respective mythologies, a relationship between a werewolf and a vampire knight is seen as taboo. Fanfictions explore this idea by creating stories where the two characters fall in love despite the odds. It adds an element of drama and romance to the plot. Another theme is power struggles. Both werewolves and vampires have their own hierarchies and power structures, and when they cross over, there are often battles for dominance and control.
In Vampire Knight fanfics, a 'bloody nurse' could be a really interesting and unique character concept. It might add a new layer of horror or mystery to the Vampire Knight universe. For example, she could be a nurse in the school's infirmary who has a dark secret related to the vampires. Maybe she's secretly helping the vampires in some unethical medical experiments, or she could be a vampire herself, using her position to get closer to her prey.
Well, a fanfic about Zero and Kaname from Vampire Knight could explore their complex relationship. It might delve into their different natures as a vampire hunter and a pureblood vampire. Maybe the fanfic shows how they could overcome their initial enmity and form an unexpected alliance or even a deep friendship, given the circumstances in the Vampire Knight universe.