A young vampire who is new to the vampire queen's court. This vampire is naive and still learning about the ways of the vampire world. He could provide a different perspective and also be a source of trouble as he makes mistakes and the vampire queen has to deal with the consequences.
Obviously, there would be the queen vampire, his mother. She could be a powerful and mysterious figure, perhaps with her own motives and plans for Naruto. There might also be some vampire elders or nobles who play a role in Naruto's life, either guiding him or trying to use him for their own purposes.
Well, of course, there's the little vampire himself. He could have a best friend who is a human, a brave kid who is not afraid of the vampire's nature. There might also be an old vampire mentor who teaches the little one about vampire traditions and how to control his thirst.
Obviously Naruto himself and Queen Celestia as his mother. There could also be characters from the Naruto series like Sasuke and Sakura. They might come to the world where Queen Celestia rules to help Naruto or to explore. And from Queen Celestia's side, there could be her loyal subjects, like the ponies if it's related to the My Little Pony world where she is from. They could interact with Naruto in various ways, either being friendly or a bit suspicious of this new 'son' of their queen.
Naruto and Shinso are the primary ones. But we could also have an old and wise vampire elder as a main character. This elder could have knowledge about a prophecy that involves Naruto and Shinso, and his actions and decisions would drive a lot of the plot as he tries to either fulfill or prevent the prophecy.
Luna Lovegood could be a crossover character. Her unique perspective and belief in strange magical creatures could translate well into the Vampire Diaries world. She might discover new supernatural beings in Mystic Falls that she had only read about in her own world. And then there's Klaus from the Vampire Diaries. He could be a wizard in the Harry Potter world in the fanfiction. His power - hungry and manipulative nature would make him a formidable dark wizard, causing chaos in Hogwarts with his vampire - like thirst for power.
There could be the last royal vampire themselves, of course. Then perhaps a loyal servant or advisor who has been with the vampire's family for generations. A hunter who initially wants to kill the vampire but then realizes they may be the key to something bigger is also a great character.
A mysterious transfer student could be added. This student could have a hidden power or a special connection to the harem queen's past. For example, she might be from a long - lost clan that has some sort of prophecy related to the harem.
Dr. Eggman could still be a major villain. He might see Sonic's new vampire state as an opportunity to study and use it for his own evil plans. Maybe he tries to create an army of vampire robots. Another potential villain could be an ancient vampire lord who sees Sonic as a threat to his rule over the vampire world.
Donald and Goofy could still be his allies. Despite Sora being a vampire, they might see that he's still the same Sora at heart. Riku could also be an ally, as he has always been there for Sora in different situations. Maybe even some of the Disney characters, like Mickey Mouse, who is known for his kindness and understanding, would be willing to help Sora control his vampiric side and fight against any threats that come with it.
A human could also be a love interest. The Salvatore sister might be intrigued by the simplicity and innocence of a human life. This relationship would be complex as she has to hide her true nature from the human, and also protect him from the other supernatural beings in Mystic Falls.