Probably not a new idea.
I'd like the players to play themselves, as they are in the world as we know it. The campaign would begin with the players playing a DnD session.
Then the proverbial faeces hit the fan. A major catastrophy has broken out. TV and Radio work for another 3 or 4 hours, before they produce nothins but static noise.
They are together as a group, and their "task" is simply to survive. It doesn't take long before the first riots break out. From here on society crumbles with the day.
I would let them play as long as possible in this world gone mad.
When the last of them dies (which inevitably will happen) I want to take the world forward for 250 years. Then it becomes a sort of post-apocalyptic story in which the PCs have to find out what went wrong.
It isn't worked out as of yet, but this game will take place!