Sakura could be a great choice. She's strong - willed and has powerful medical ninjutsu skills. In the Vampire Knight world, her medical skills could be used to help both humans and vampires alike, or she could use them to fight against vampires if they pose a threat. She could also hold her own in any confrontations due to her combat training.
Zero could be one of Naruto's allies. They both have a sense of justice. Zero is fighting against his vampire nature, and Naruto could understand his struggle as Naruto has fought against his own inner demons. Yuki could also be an ally. She's kind - hearted, and Naruto's friendly nature might draw her in.
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.
Ichigo from 'Bleach' could be an interesting choice. His sword skills and his own unique powers could be combined with the sealing techniques in Naruto. He could learn about seals from the Naruto world and use them in his battles against Hollows. Maybe his Zangetsu could have some sealing abilities added to it.
Naruto and a wise, old swordsman. Naruto's energetic and determined nature would contrast well with the swordsman's calm and experienced demeanor. The swordsman could teach Naruto how to use a sword in combat, which Naruto would then incorporate into his own fighting style in a unique way.
One possible plot could be that the female knight from the 'Goblin Slayer' world gets transported to the 'Naruto' universe. She teams up with Naruto and his friends to fight against a new threat that is a combination of goblins and some of the villains from the 'Naruto' world, like Orochimaru who has somehow managed to use goblin - like creatures in his experiments.
Luffy from One Piece could also be a good crossover character. Luffy's Gum - Gum powers and his carefree yet determined attitude would be an interesting match for Naruto. They could be on a quest together in a strange new world, with Naruto using his ninjutsu to solve puzzles and Luffy using his stretchy body to reach places that are otherwise inaccessible. Their different personalities would also lead to some humorous and heartwarming moments.
Naruto's chakra - based abilities, like the Rasengan, would be a huge shock to the Vampire Knight characters. They have never seen such concentrated energy attacks. Meanwhile, the vampires' superhuman strength and speed could be a challenge for Naruto at first. But he could use his shadow clone jutsu to outnumber them. For example, if a vampire tries to attack him with speed, he can send out clones to surround and counter - attack.
One idea could be that Naruto is somehow transported to the Vampire Knight world. He might initially be seen as a threat due to his unknown powers. But then he could start to form alliances with some of the vampires or the Day Class students. For example, he could team up with Zero to fight against the more evil vampires who want to harm the humans at the school. His ninja skills would be a new and exciting addition to the Vampire Knight universe.
One popular 'naruto vampire knight crossover fanfiction' is 'Naruto in the Vampire Knight World'. It has an interesting plot where Naruto is transported to the Vampire Knight universe and has to adapt to its rules and deal with the different factions.
Sasuke could be one of his allies. He might be a rival knight at first but then they realize they have to work together. Sakura could be a healer knight, always there to patch up Naruto and the others after battles. Kakashi could be a wise old knight who gives Naruto advice on knightly matters.