For whatever reason, civilization as we know it is about to come to an end. You know it, you see it coming, and you try to warn people, but no-one will listen. So you take matters into your own hands.
The game will be broken into two parts: Pre and post- apocalypse. In the first half, you must go around the world | land | continent and gather seeds and nuts to stockpile for re-planting after the world ends. You must find a minimum variety of seeds, and a minimum quantity of each species.
Cue the apocalypse. You go inside and spend an entertaining x number of months/years protecting the seeds from those who would eat them...or you.
Finally the apocalypse is over. Time to go out into the barren wasteland and try to re-start the ecology. With luck, you still have enough seeds to help civilization get back on its feet.
There are many details I could throw in, like a time limit, being the countdown to the end of the world, then another countdown once you re-emerge, trying to get the first crop in before it is too late in the season, or before you starve.
Special treasures could include various kinds of fertilizers, or hydroponics equipment, or water reclamation facilities. The first half of the game could include simple economics as you try to fill out your collection by trading with the people around you.
I would want a lot of the content and landscape to be procedurally generated, just to keep it from being the same game every time. With the research I did for the game last year, it shouldn't be too difficult to set up a system of some kind.