A Cthulhu Mythos game that takes place in a later time period than the original stories, say, the '50s or perhaps even the '70s.
Or, a Cthulhu Mythos game series taking place in several time periods, starting with the '30s, all in black-white (or perhaps sephia), and having a monochrome sound. Then, in the '50s, it shows everything with a somewhat correct coloring, but not quite (the practice of coloring black-white footage). This is still the era with monochrome sound.
We then go to the '70s, where we now have stereo sound and full color. However, like the previous two eras, it does have a grain filter over it, although the grain is significantly less than in the previous eras. Also, sound quality isn't optimal.
Then, we skip to the '90s. No grain filter, audio is almost perfect, stuck in 44100 Hz, and stereo only. Colors are brighter, to represent the fact that it was a sort of brighter era (Baywatch, various summer hits having music videos with similar styles, the Will Smith era, you know, all those typical stuff).
Finally, it's 2010. Everything is dark again, because
real is brown. Audio is now 5:1, perhaps even upped at 48000 Hz or whatever is popular nowadays. Perhaps have some bloom filters, as it's typical of realistic games.
And, all throughout the entire series, anything supernatural will look out of place. In the '30s, the creatures seen will have a color tint that really looks weird, like, a bit of green, or a bit of blue. The '50s will have the sounds of the creatures be in stereo instead of mono. I don't know what would make the '70s creatures out of place, perhaps make its sounds better quality, while the grainy filter temporarily disappears. Then the '90s has the monsters scream in 5:1, and finally, 2010 will have some other things I can't think of.
Seriously, this could have a lot of potential.