I've played around with this a couple of times, in a couple of different ways. Currently, my "doodle work" (i.e., the writing I do to get away from doing my work-a-day writing) is exactly such a campaign. I use the basic premise that it the setting takes place after the Change, where magic returned and technology failed. The elves and dwarves and such have returned, monsters roam the land, and so forth. I stole this basic idea from Steven Boyett (
Ariel,
http://www.amazon.com/Ariel-Steven-...4/ref=pd_bbs_sr_1/104-4459848-9850363?ie=UTF8) and S.M. Stirling (
Dies the Fire,
http://www.amazon.com/Dies-Fire-Roc.../ref=pd_bbs_sr_2/104-4459848-9850363?ie=UTF8). I'm toying with two eras, one about thre years after the Change, when young adventurers go forth into the Changed world to discover what happened to the rest of the country, and another about 1,000 years later, which is really more of a latter-day Horseclans (
http://en.wikipedia.org/wiki/Horseclans) style adventure.
The best maps for this kind of game are from DeLorme (
www.delorme.com), you can find them at any Barnes & Noble and most other bookstores. For a campaign set right after the Change, they are perfect; anything urban is essentially wasteland, filled with the bones of the dead, and perhaps undead, and PC's will be based out of small farming villages way off the beaten path. For a campaign taking place later down the road, you can match up bits of forest and such to create new wilderness lands, and use classic medieval demographics to determine the current population and such (
www.io.com/~sjohn/demog.htm).
You can d a lot of interesting things culturally in a post-Change world, which is essentially a magical post-apocalypse setting. For example, in the 1,000 year+ version that I'm working on now, the lands in the triangle between Chicago/Detroit/Indianapolis are known as "Mooristahn," a black pseudo-islamic region lifted almost directly from the Horseclans setting's "Zahrtogah," though as it has the "old gods returned" the religion is a mix of Egyptian, Babylonian/Sumerian, and Phoenician (actually, I use the gods and pantheons as outlined by Gary Gygax in his Lejendary Pantheons). Arabian Knights meets weird fantasy in the Midwest, and I can use the maps straight out of the modern atlas, with a few additional bits of wilderness. Counties are beyliks, and several counties make up an emirate, malikate, or even sultanate. The same is done elsewere with other cultures and regions, i.e., each county is a county or shire, and several together are a duchy or kingdom or republic or what have you.
For macro-politial divisions I used the maps from here:
http://www.cpc.ncep.noaa.gov/produc..._DIVS/states_counties_climate-divisions.shtml
They aren't always based on geography, but they make things easy. Each region is a kingdom or sultanate or duchy or whatever. There are tons of cool resources like this out there...