Devil's advocate: parts of La Noscea (and also the Dravanian Hinterlands!) already fill the plains-style-geography niche that a Green Coerthas would fit into, and Snowy Coerthas makes for a more distinct region vis-a-vis the rest of Eorzea.

I wouldn't mind see the Eastern Highlands and the Lowlands at some point, though. It might be nice to see the transitional geography between them and Mor Dhona/the Shroud.