Another plot could be about the little vampire's family. There could be a conflict within the vampire family, like an old feud with another vampire clan. The little vampire has to navigate this while also dealing with his own growing up in a 3D world, which has different rules and challenges compared to the traditional vampire world he's used to.
Perhaps the fanfiction could explore a friendship between the little vampire and a human who discovers his secret. They could go on adventures together, with the human helping the vampire understand human emotions better, and the vampire using his powers to protect the human from danger.
One idea could be exploring the vampire's family history more in - depth. Maybe there are ancient secrets or rivalries within the vampire clan that the main characters could uncover.
A plot could be Doris discovers a hidden vampire lair near her village and decides to take matters into her own hands before D arrives. She could use her knowledge of the local area to her advantage and try to set traps for the vampires. This would show her courage and resourcefulness.
Perhaps Rudolph discovers a hidden power within himself that helps him protect his vampire family from a new threat in their town. This could involve a series of challenges where he has to learn to control this power.
One idea could be the little vampire befriending a human who is also a slasher movie fan. They bond over their differences and similarities, like the vampire's nocturnal nature and the human's love for horror. The vampire could show the human the 'vampire version' of horror, which is their everyday life.
A really interesting plot could be that Gregory gets lost in the human world during the day. He has to find his way back home while avoiding being discovered by too many humans. He could meet a kind old lady who helps him without really knowing what he is, and through this encounter, he learns more about human kindness.
One idea could be to cross Vampire Hunter D with a post - apocalyptic world setting from another series. D could be on a quest to find a vampire lord who has the key to a cure for a virus that has decimated humanity. He would face both vampires and mutated creatures along the way.
A crossover with "Castlevania" would also be cool. D could find himself in the world of Castlevania, dealing with the various dark creatures there. He might have to navigate the complex politics between the vampires and the humans in that universe.
One idea could be to write about the little vampire's first day at a human school. He has to hide his fangs and his love for blood while making friends. It would be full of comical situations like when the cafeteria serves steak and he has to resist the urge to pounce on it.
One idea could be a crossover with a popular wizarding world. The little vampire could attend a magic school and have adventures with the wizard students, facing magical creatures together. Another idea is to cross it over with a time - traveling story. The vampire travels through different eras, experiencing different cultures and historical events.
Maybe the OC has a special power or ability that Rudolph needs. For example, the OC can sense when danger is near. They team up to protect Rudolph's vampire family from a group of hunters who have come to town. As they work together, a deeper bond forms between them.