This is an idea that has been, admittedly, on my mind for a while. I've only recently really put some thought to it, and think it'd make an awesome group game. I'd run it myself, but with two fairly large groups, both close to BGS, that I'm running, I don't think I could do it alone.
What I had in mind was this...
A survival, post-apocalyptic story that's somewhat in the vein of the Walking Dead and Stephen King's The Stand.
Two factions, two rival groups/civilizations.
One Good, chosen by Angels (unknowingly), and one Bad, chosen by demons (unknowingly). I thought they could know of each other, have some sort of uneasy truce, in a world devastated by an infection that has killed most of the population, while they secretly plan for each other's demises. Maybe some of them are secretly Angels or Demons (ala Randall Flagg).
I thought maybe the Good live in a nicer community, one that is more strong together, while, maybe say across town or a few miles away, lives the Bad, in a sort of more lawless group that sees strength in numbers. The Good cling to the way of life before it all changed in the infection, and the Bad sort of see it as a dog-eat-dog, survival of the primal type of thing.
I wouldn't mind co-GMing it or doing the world building with/for someone who is a capable GM, one who wouldn't run it into the ground or just put everything on my shoulders. I think there's a lot of potential here for a great game, lots of character development, and a whole lot of fun.