Perhaps the unknown civ will have the same culture group as Americans, Aztecs, & Iroquois. Through PTW, those are the only three in that culture group. For comparison, the northern european one has Scandinavia, England, Russia, Germany, France, and Spain, the southern European one has Rome, Greece, Egypt, and Carthage, the middle eastern one has Ottomans, Persia, Arabia, Babylon, and Zululand, and the east asia one has India, Mongols, China, Korea,and Japan. The Celts belong to either the northern or southern european group, but since I haven't played as them, I don't know which one they are.