I think this may be BL's first shot at a single-player mode.
You should be a survivor of a cataclysmic event that involves otherworldy creatures, hellish and disgusting creatures, popping out of thin air and attacking people. I know, but it would be like a survival horror game. You survived after hiding in your apartment for weeks on end, subsisting on what's in the fridge. When you go outside, you see a man, but he's possessed by the hell spawn that destroyed your city.