This idea has been gestating in my brain bucket for a while now. The following is just a basic overview, to gauge interest, so don't expect a full fleshed out world just yet.
For centuries Vampires lived in the shadows of human history, concealing their presence through a series of strict rules. Once such rule was: Never drink directly from a Human. One day an arrogant child of the night decided to break this rule, and consequently brought human civilisation to an end. He passed on to his victim a virus that would come to be called "Hemophagia". Humans bitten by vampires become a twisted mockery of the creature who attacked them, and would spread the disease to their own victims. Mindless animals, driven solely by a never ending hunger for blood, the Hemophages, spread like a plague, with every human bitten succumbing and swelling the ranks of the blood drinkers. Not only was this plague bad for the humans, but meant doom for the vampires as well. Hemophage blood was toxic to vampires, meaning with every human that turned, their stock of food dwindled further. Now, several years later, the Hemophages rule the world. Pockets of humans survive by hiding during the night, and scavenging for supplies during the day when the Hemophages retreat into their dens to avoid the sun.
So this is similar to a zombie rp, only with feral blood drinkers instead of shuffling flesh eaters. The main enemies are, of course, the Hemophages, who are not immortal, vulnerable to UV exposure, and can be killed with only slightly more difficulty than one kills a human, but Vampires will also present an occasional threat, being dangerously intelligent, superhuman immortals that can only slain via beheading, destroying the heart, or UV exposure. For the most part I expect it to be more or less open world, though I do have some ideas about maybe a race to secure a cure to Hemophagia, where survivors have to get the cure before a group of Vampire do, who want to use the cure to cultivate their own personal stock of hemophagia-free cattle.
Open to thoughts and critiques, or even just a declaration of interest.