Divine Wind does this already. It add Wastelands areas that cannot be colonize or crossed but are explorable.
Um I mean a mod that allows colonization of those regions... Im not really too fussed about the really northern ones etc... But there is a large chunk of USA which apparently is too inhospitable to colonize even though lots of people leave there lol...
So yeah looking for a mod that has those regions as actual regions... Colonizable and buildable etc...