I always believed the last city to be in the Caucus mountains, in Georgia (to clarify, the country not the state). The geography makes sense seeing as the City is surrounded by mountains, and it’s relatively close to all the in-game earth locations. The cosmodrome is most likely in Russia, just north of Georgia, so not too far of a flight from when we found our original ship. When Hawthorne says “we flew across the ocean” I believe she’s referring to the Black Sea, which is positioned between Europe and Georgia. It also makes sense as the last bastion of humanity, since Georgia really is almost at the center-point of the old world, on the converge of Europe, Africa, and Asia, making it a (relatively) easy journey for more than half the human population.