I have played Fallout 4, awesome game, and my first ever Fallout game aswell.
My game set in the Fallout universe, gosh if i do that Bethesda won’t be too happy with me, my idea involves staying in the universe we live in today, also i only just realised, i’m starting from World War 3, and i bet someone is going to ask me “Why don’t you start from World War 1, and make your way to World War 3?” In fact i’m doing up my design documents for World War 1 and World War 2, needless to say i may have to split World War 1 into 4 parts, each part covers one year of World War 1, with World War 2 i would have to split into 6 parts, though as you noted earlier, gotta get the main game built first, once the main game(or base game as i like to call it) is done, i can then add all the finer details like story, setting, etc.