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.
Perhaps there could be a character in the Harry Potter world who has vampire - like abilities and is mistaken for a real vampire. D is called in to deal with this 'vampire' but realizes it's not a normal vampire case. He and Harry then have to figure out what's really going on. It could involve a dark wizard's experiment gone wrong that created this half - vampire - half - wizard creature, and they need to find a way to reverse it or at least keep it from causing harm.
Alucard from "Hellsing" would be a great addition. His overpowered nature and complex personality would contrast well with D's stoicism. They could have an interesting dynamic, either as allies or enemies depending on the plot.
A fanfiction might focus on Doris' relationship with the people in her village. It could show how her life was before D's arrival, her daily routines, and how she was seen by the villagers. For example, perhaps she was a bit of an outcast because of her family's past, and this could add more depth to her character.
One possible plot could be that Vampire Hunter D and Van Helsing team up to hunt a powerful vampire that has been terrorizing both their worlds. D's unique dhampir abilities combined with Van Helsing's knowledge and skills would make for an exciting adventure.
A powerful ancient vampire lord could be the main antagonist. He could be so old and strong that he has knowledge and powers that both D and Van Helsing have never faced before.
The theme of mystery is quite prevalent in 'vampire hunter d fanfiction'. D's past, the origins of the vampires, and the strange powers and artifacts that are often involved in the stories are all shrouded in mystery. Another theme is survival. D has to constantly survive in a world full of dangerous vampires, and the humans he tries to protect also have to fight for their survival. And the theme of the supernatural is of course a given, with all the vampire lore and D's own unique abilities.
Yes, there are some really interesting 'Vampire Hunter D Castlevania' fanfictions out there. You can often find them on fanfiction websites. Some might focus on crossovers between the two universes, like having D interact with the characters from Castlevania. Others might explore new storylines inspired by the unique elements of both.
Friendship and loneliness are themes as well. D is often a solitary figure in his hunting, but sometimes he forms unexpected friendships or alliances. Some fanfictions explore how these relationships affect D and his mission. There is also the theme of the unknown and the supernatural world that D is constantly exploring, which adds an element of mystery to the stories.