I really, really doubt that there is anything underneath the clouds on the southern part of the map. It just looks like inaccessible mountainous land, which we probably will never get information on regarding exactly what lies in that area. The same has happened numerous times before, with the northern-most parts of Kanto, Johto and Unova still being unexplored.
I think that although the region map looks small, it's not at all proportional to the perceived scale and detail which will be present in the game. Lumiose City looks massive in screenshots but looks tiny in comparison to its appearance on the map.