If it hasn't been suggested in the last few pages, and Yahtzee would be up for collaborating with a load of internet wierdos (and maybe having to change the title of Actually Scary Game), it would be right interesting to see what treatment he'd make of SCP Foundation: The Game.
It's probably quite weaksauce horror compared to what kind of primal fear stuff he's thinking of, but it gave this self confessed horror-pussy serious mindfuckage being alone in a quiet, dark house after several evenings of noodling through their articles. There are be several very rich veins of creep-out waiting to be mined within said archive, even if sturgeon's law is ruthlessly applied. The one concerning a device that lets you pass through a mirror into some parallel, post-space-apocalypse/holy war/kafka-meets-giger cataclysm earth (with the destination changing based on the holder's karma) would alone make for a considerable and complex subgame.
Also mining the annals of gurochan's /f/ board could bring serious results. I know that on the "artistic thread" there at the moment there's some seriously fucked up shit, including a picture whose thumbnail alone chills me slightly, because I've seen the full size one before and ... well, the artist must have some kind of direct line to the brain's terror centres because they created a still image, from oil paints and animal hair brush, that damn near made me crap myself. The Alt and F4 keys of the keyboard I was using probably still bear the scars of me panic-quitting the browser with unreasonable force when my mouse hand froze. Closest thing yet experienced to the "basilisk" image... all that from an arrangement of pixels... crikey. It's embarrassing is what it is. Imagine the power to spread said terror and subsequent embarrassment on a global scale.
Also, even though the quality of FSG would probably be excellent, the world can probably wait longer for a highly polished Asteroids clone than for something offering true ball-shrinking scares, taking silent hill and eversion into an ex-nazi science camp experimentation room and blending their freak-DNA in some kind of occult ritual and adding plenty of other ingredients for spice.