Havnt had time to read any of the above, but heres my 2 cents.
How about we take the idea of vampires, dehumanise them a fair bit, make them be lithe, pale humanoids with fangs and claws and glowy red eyes, call them some diff name like Nosferatii, Nocturne or Vampyrii, and make them be a secretive race that has the ability to infect other races with a blood born parasite that (if the infected survives) makes the infected similar to a vampire (possibly like a fantasy ghoul style vampire, you know, vampiric but in a degenerate way, could even make them zombie like but fast like ghouls).
The vampire race would be naturally stealthy, and brutal in close range firefights and close combat.
They would feed on the nutrients in other creatures blood, have regenerative abilities and high speed.
Make their look be a kind of mix between traditional vampire gothic and sci fi, but more sleek and alien than the usual 40k gothic style (but not too un 40k).