One common theme is the forbidden love aspect. Since D is a dhampir and there are so many differences between him and a normal human (the reader), their relationship is often seen as something that goes against the norm.
Sure. You can check out fanfiction platforms like Archive of Our Own. Just search for 'Vampire Hunter D x Reader' there, and you'll find a variety of stories with different themes and plotlines.
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 interesting story could be about a reader who discovers a unique bond with the Hunter. Maybe the reader is initially scared but then realizes the Hunter has a certain protectiveness towards them. It could involve the two of them surviving together in the post - apocalyptic world of L4D.
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.