I don't know if Emer III will also describe the southwest; if not, I would like the last Emer book to close the circle.
Apart from that, personally I prefer some kind of more focused supplement; Emer I, II and III are too general and it's a good start to get the flavour of a continent, but you can find that local maps are somekind 'empty'. Decreasing the scope you get a more detailed environment. Jaiman still hasn't a specific setting for Urulan, northeast (Zor, Jamil Taarg (sp?) or southwest (U-Lyshak,Ly-Aran, Ulor and Saralis) for example.
Another small supplement I'm thinking about is a races & cultures for Kulthea with all the stuff you need for a complete background: languages, localizations, special training packages, adolescent skill ranks, martial arts for MAC, priests spheres for deities (CC), etc.