A bit of alternative history here: Instead of creating a runaway greenhouse effect, humanity has created a runaway albedo effect, perhaps caused by the lighter patches of earth left behind by deforestation, or the tons of metallic space-junk orbiting the Earth and reflecting sunlight away from the planet, or possibly even the massive feats of geo-engineering intended to reverse the warming trends of the planet. However the Great Freeze was caused, it started slowly and accelerated alarmingly. Ocean levels plummeted as the ice sheets crept farther out across the Earth than they had in around 12,000 years. 
The cooling created droughts. Crops withered. The poor were the first to die in great numbers. The gluttonous rich died next, and the last, many of whom are apocalypse prepper types, have managed to survive. Society collapsed after widespread riots erupted in most major cities. The cities had it the worst; as food got more and more scarce, people began to kill each other, and the crime rates rose so high that the overstuffed prisons couldn't hold anymore people. 
[img]http://media.web.britannica.com/eb-media/18/64918-004-1078906E.gif[/img]
I'm not sure what time exactly in the ice age the map refers to (as in years ago), but at the point in the time of the roleplay, they're more or less in that location in the United States. The roleplay will take place in the northern or midwest US, but not on the ice sheets, as those are barren wastelands that spell certain death for any human that wanders too far north (Although there are probably isolated societies of people who have learned to survive in those conditions, but they're even more spread out than the city-states). In many places, the ruins of old cities lay dead, looted and picked clean decades ago; nature is starting to take a hold on them again. Animals make their dens occasionally in collapsed buildings, and tough trees take hold in the cracks of sidewalks, eventually growing big enough to break apart the pavement. 
Farther south, there are a few city-states, few and far in between, where agriculture remains a part of life where the temperatures are a little more tolerant. To the north, people have reverted to hunting and gathering and occasionally stealing; they travel in groups of around 20 or 30 and follow their food wherever it may go. 

Good idea? I sure hope so. Maybe I'll establish a city-state and a hunter-gather group, and they can possibly interact or get in fights or something. Comment if interested of course, and please give ideas on setting(s). I'm thinking of having it set a bit east of the Cordilleran ice sheet, so somewhere in Colorado.