I'm going to use Raylib to make a VR game where you are stuck in a semi procedural-generated underground facility that is being overrun by genetically engineered slime mold. 

No enemies as such, just the constant knowledge that this thing is spreading everywhere that you aren't fighting it back, cutting off access, stopping machinery from working, entombing you.

It'll have some semi-realistic behaviors too, taken from actual slime mold "pathfinding" it'll initially send out a very thin covering of itself but upon detecting food it'll rapidly increase the amount of itself present in the area, generating thick veins through which it courses. 

Through this method I can create a situation where the enemy is always present but not always a full threat, where you aren't being actively chased but don't want to hang around in one place too long. 

Oh and it'll be full of ebin may mays and collectable hats.