Here's some ideas off the top of my head that might be fun:
-Wild brahmin herds, big nomadic offspring of Montana's current day cattle ranches led by buff ass "bullmin" who constantly clash over grazing territory
-Big ass yetis from North Dakota to Minnesota living off the unkempt moose and deer populations
-Lake Superiour's levies were all sabotaged and Wisconsin is partially flooded, Chinese spy feral ghouls in the dams' mechanisms from the Great War
-Concrete Jungle, trees growing through the sidewalks, everything just picked clean in Michigan since it's already been looted and left for dead as is. Ironically barren in its lushness. The only actual usable stuff is owned by the outcasts of the yetis who wear car parts as armour and carry axels as clubs