Stuck on Setting

So I've written a pretty decent game about action heroes that's about halfway done when I wanted to put in a setting. I've got two ideas: one where the cold war basically decimated South American society and created an unstable breeding ground for terrorism and drug running, and one where the players are the special forces for the New World Order and assist in black ops against the kinds of people who resist the New World Order, in kind of a shadowy spy vs spy sort of way.

But see, the problem is that my game is explicitly designed for one-shot adventures, and I'm finding that the more time I spend creating an interesting and detailed setting, the more it feels like the game should be more episodic and long-term. The more the setting is treated like a character in its own right, the more you'll naturally want to see players interact with it and make things happen, which means that the players are going to be involved in a long-term game where they make the world a different place.

So I'm stuck between basically creating a game with no setting and creating a setting with a game that doesn't support it, and either way I feel like I'm about going crazy.