While no game is perfect, I've had an idea that would be fairly interesting in my mind.
So, you start out with a US that has encountered some horrible problem and collapsed, but while the population dwindled, society has survived to a degree. This would take place on the west coast in and between Los Angeles and Seattle, both of which are the largest cities, and have industry that depends on each other to thrive. This is where you come in.
The idea is it's a very large free roam racing game, based around package delivery from one point to another. Because LA to Seattle would be ridiculous as the only destinations, I can imagine you'd have towns and cities along the way as trading hubs, and you'd do missions between these places.
The system would also be fairly modular, being able to modify the array of cars, or even going down to such a scale that you'd choose a basic chassis type, then add modules as you please, such as engines (Mainly diesels, gotta have that fuel economy for long distance) suspension types, or cargo capacity. You could have basic classes like buggies, trucks, rally car-esques, or for a different feel, a motorcycle or ATV.
In order for the game to be fun and remain fun, the driving physics have to be realistic, but forgiving, and above all, tight and fun. It's got to feel like the stuff's got weight to it. Heck, if it were possible, you'd turn it into an MMO and just have tons of people running supplies around in an immersive world.
Anyway, this idea was spawned from playing Fuel, and a bit of inspiration from Fallout New Vegas.