Just going from Video Footage and general large map stuff (in-game), I'll make a list of areas within each region. Any Regions and Areas not in video footage are left out (for the most part)
East Coast (Give or Take):
- NEW YORK
--- Midtown
--- Greenwich Village
--- Chinatown
--- Financial District
--- Brooklyn
--- Lower East Side
--- East Village
--- Little Italy
--- Upper East Side
--- NY Airport
--- Central Park
-- Bayonne (Actually NJ)
-- Jersey City
- Long Island
- Cape Cod
- White Mountains
- WASHINGTON
- Coal State
- Ocean State
- Allegheny Forest
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The Midwest:
- Amish Country
- Lake Erie Shoreline
- New Holland
- INDIANAPOLIS
- Bluegrass Region
- Central Plains
- ST. LOUIS
- CHICAGO
- DETROIT
- Industrial Harbor
- Dairyland
- Mother Road 66
- Kansas Sunflower Fields
- Dakota Grasslands
- Lower Peninsula
- Badlands
- Michigan State Forest
-Central Hills
--- Black Hills
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Mountain States:
- Yellowstone
- Salt Lake Resorts
- Jackson Hole Valley
- Glacier National Park
- Pikes Peak
- LAS VEGAS
--- South Suburbs
--- West Commercial
- Red Rock Canyon
- Lake Mead
- SANTA FE
- SALT LAKE CITY
- Death Valley
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The South:
-MIAMI
--- South Beach
--- Biscayne Bay
--- North Bay
--- North Atlantic Ocean
-NEW ORLEANS
- Gulf Coast
- Old Louisiana
- Everglades
- West Florida Coast
- Keys (Florida)
- Orange Grove
- Great Plains
- DALLAS/FW