Vampire Hunter D is the future of the Hellsing universe.After Integra's death, Alucard "slip his leash" and decided to conqueror the world under his real name, Dracula. He succeeds, establishes a kingdom of vampires, but becomes disgusted and annoyed by his childer's behavior. Que a massive purging by old Alucard which leads to downfall of the vampire ruled civilization, causing a new dark age. At some point, either before the purge or during it, Alucard has a tryst with a mortal woman and the result is D.
Doris Lang and Leila are related.The two of them have the same eye and hair color and, though there are significant artistic differences between the two movies, appear to have the same sort of body type and facial features. Since there's not a specific time as to when the stories are set in relation to each other, this could shake out two ways: Either Doris is the young girl that talks to D as he puts flowers on Leila's grave at the end of Bloodlust, or she is Leila's mother, who was turned by vampires and eventually murdered by the townspeople, inspiring Leila to become a hunter.
Dracula is really a Tragic MonsterAnd D will die along with him, after they both save the world and set it back on the path to restoring itself.
The world D lives in is a possible future for the Monster Hunter world.