One huge surprise that wasn't revealed until BO3 was out in the wild is that completing the game's vanilla campaign mode will unlock Nightmares, a zombified version of the campaign which sees you playing through the levels again with a new story, replacing all those robots with the blood-thirsty undead. In many ways Nightmares is much more satisfying than the original campaign, serving up a simpler but better-told story which repurposes what came before in an interesting and clever way. It's clear that Treyarch put a ridiculous amount of effort into this, but now the only problem is that fans are going to expect every CoD game to boast some sort of post-game remix like this. Hopefully Infinity Ward are already hard at work on it...