That this could be solved by making each sub-zones of the big regions different but still using the same eco-system theme like the drawing of the OP express
http://seahorror.com/thoughts.jpg. So that each sub-zone would have it's own flavor and monster types and you would feel a progression by advancing to the next sub-zone.
Would be like combining Saruta, Taronhgi, Giddeus, Buburimu and the dungeons like the maze of shakrami and the towers un Saruta to be under a "seemless" region formed by all these different sub-zones with their own flavor but all with the same type of eco-system in the background.