Quos Deus vult perdere, prius dementat - The insanity causing properties of the objects and entities of the Cthulhu Mythos are a a result of the Great Race of Yith (and/or Yog-Sothoth and his minions) liking their privacy. As masters of time it is in their power to arrange things so humanity at large is fated to never learn their secrets. As a result events in general conspire for people to never encounter them, HOWEVER some coincidence always happens to discredit anyone who does stumble upon the secrets of the mythos, such as suffering from some damage or illness of the brain that renders them insane. The increasing world population as well as the proliferation of digital cameras and telecommunications technology means that they are more frequently seen, more frequently photographed, those photographs are more frequently shared before some coincidence can cause them to be lost, and more people have the resources at hand to deduce their existence. The result of all this is bizarre events of the past decade of so and the world's continued descent into choas and madness. Cut to the not too distant future, where in the waking world civilization is being torn apart by a pandemic of violent psychosis (akin the the start and end of the film In the Mouth of Madness) while at the same time Earth's dreamland is collapsing in on itself and Nyarlathotep reigns over it unfettered. This is the world in which the player characters find themselves, and there is no saving it, the arbiters of fate have already spoken. The most they can hope for is to survive as long as they can. At the very best they might help to attenuate total destruction into a mere fall into a new dark age or might escape through the collapsing dreamland to seek refuge witb the gods of Teloe (see The Crawling Chaos)
Blood Sacrifice- The PCs are vampire hunters. They are in Mexico or central america to hunt down the ancient vampires who were the secret puppetmasters behind the Aztec and Mayan religions.
Deathboat- Zombie virus outbreak on a cruise ship out at sea
Miscallaneous ideas:
*20000 Leagues Under the Sea, except in space
*Arthur Gordon Pym, except in space
*a cross between Ghostbusters and Pokemon. catching ghosts to fight other ghosts.