Vampire Hunter D/WMG

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.
 * According to the novels, humanity brought on its own ruin through a nuclear holocaust, and the vampires stepped in when they decided that they would be much more worthy rulers of the planet than the humans who nearly destroyed it, and the end of the vampire civilization was caused by the hubris of immortal creatures who had seen and done it all, and lost most of their vitality and drive; a slow decay that took thousands of years to culminate in the new rise of humanity. Though it doesn't necessarily mean that this WMG is without merit - just that it needs to be revised a bit on the details.
 * Possible, considering D looks suspiciously like Alucard and wears basically a blue and black tieless version of Alucard's outfit.

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  or she is

Dracula is really a Tragic Monster
And D will die along with him, after they both save the world and set it back on the path to restoring itself.