This is a good theory, upon thinking on this thought I have come to a conclusion, if this spreads like any other disease then the higher concentration of people the better/faster it will spread. In such case we can look for regions that are not only completely infected but with varient versions and infected quickly, helping us narrow it down. Further more with our knowledge of the Topography of the Last City's location we can place it in a cyclonic or polar climate. Lastly we can use previous theories of where it may be as there was the evidence there still stands. Using this information as a filter we can bring it down to three places, The Ural Mountains, The Rocky Mountains, and The Kolyma Range. Those who can connect any of these places to lore or further narrow it down with evidence I may have missed.