"An apocalyptic event occurs--lets say a world-altering nuclear accident on the West Coast, just for an example.
North America has been depopulated except for the East Coast--it's say back to the original 13 as they existed in the 1750s. So, Virginia essentially ends at the Blue Ridge, Maryland at Frederick, Pennsylvania at Carlisle, etc... Except for isolated pockets of civilization all over the country.
Native American tribes have reclaimed much of the continent, etc.
You, as the player, are smack dab in the middle of everything. Let's say you start in the decaying ruins of a rust-belt city.
In this setting, the game would be very much like the feel and setting of STALKER, without the radioactive mutants and stuff--because the nuclear accident took place on the West Coast.
You have wild dogs and various small factions of humanity and native tribes.
You can move from this city to anywhere in North America--deeper into the West to find the nuclear accident site (although here you would encounter radiation and mutant monsters and stuff) or go East and visit the settlements on the coast.
There would be all sorts of factions with inter-tangling relations between them. One faction might be a part of another--like a regiment of an army: you aren't a member of Army A, you're a member of Regiment 1 or, even lower, Company Z. You'd be able to do any number of career-type paths or do odd jobs or be a beggar or whatever.
The game would really have no "point" except to do what you want. If you choose certain paths, it could result in an endgame scenario--like you advance so far in a legislature that you become Emperor of North America and then there isn't much else for you to do--you wouldn't dare walk around with your subjects because you'll probably be killed by them--besides order people around and then wait.
The focus, though, should be on the frontier. I really want this to be fluid and violent and unorthodox like the first few years of the French and Indian War, but with modern weaponry.
But your game would change quite a bit depending on where you choose to spend your time--if you go West, you'll battle mutants and do FPS stuff; if you go East, you'll do RPG stuff..."