I'm actually leaning toward the possibility that this region is based off of China, or some part thereof. Reasons including the much greater amount of people, larger landmass, large separation from the other regions (but not ridiculously far away like the United States), and the similar, yet distinct look of the Pokemon, region, and culture presented in the screenshots we've seen. America just seems like too much of a stretch from Japan.
The biggest reasons, though, were the obvious introduction of the yin/yang idea presented in the game titles themselves and the main legendaries, and that overlooked new fire type revealed today, Hihidaruma. It looks exactly like a Chinese dragon. Granted, since the cultures of Japan and China are closely intertwined, these ideas may have been pulled from the Japanese background, but the fact stands that they originated in China, which is why I'm thinking the new region is based off such. However, I don't think the new region is a cut-and-paste of a section of land that has been rotated a bit like the other regions (Kanto & Johto, Hoenn, Sinnoh). At least, I can't identify anywhere that it resembles in China, the United States, or Japan. Especially considering the proximity of mountains, ocean, and desert to each other.