Inspired by this post and TLOU2 in general:
A game that starts out like a normal FPS, but the first time you kill someone, you immediately jump into a flashback from that person’s perspective, with one of two outcomes. Either you play as that person up until the moment they are killed by the original player character, or until the person you are currently playing as has to kill someone else, at which point you leap into the past of that person, and so on, ad infinitum, until you end up at some kind of 2001 prologue sequence of Early Man bashing another guy’s head in for the first time