Well considering that since factions Anet idea seems to make chapters inspired on real cultures, lets think of war like civilizations that have clashed at each other. we havent seen much of a clash in the actual chapters, only between the Lux and Kurz so...
Think about it, Anet as stayed away of European civilizations because they wanted to be original when prophecies first came out, the most european thing we have ever seen is pre searing Ascalon, I believe its about time to use European cultures. I would personally would love to see a greco-roman culture and a meso-andean one, you know, the well known clash between european and native american civs. both of them in the same game have everything GW needs, impresive unique landscapes, impresive architecture, war like cultures, weapons, armors, lore, huge amounts of mithology and folklore, unique and ditinctive cultures, they can fit well in practically all profesions
Plus Anet has proved they can mix similar cultures and create one out of them, like Cantha veing a mix of 3 countries, and Elona being a mesh of middle eastern cultures. just imagine a Mexica-Maya-Incan culture in gw!