Good work so far.
Now then when you know that, I might have some ideas for you. Since you thinking this to be a game of this guy slaying the whole human race, my idea for is now when you said you want it to be a game with a free mode when you like say can go around the world slaying peapole and fighting militerries. And the you could have neither that you to to a place to start a misson or like take conntact with how ever you had this misson to eliminet the human tace. And depending on how big your game is susposed to be you can make choice of whice place of the world you want to be in and to do so you might have to do a misson or what so ever. And you can have a unlocking system for new missons and weapon and armor and etc. Well that's a idea to start with.