For new civilizations... The Middle East is pretty much rounded out, so we'll move on from there. Europe is pretty much set, though if a colonial scenario is in the works maybe adding the Dutch or Portuguese would be a good fit. Whichever one is chosen, the UU should probably be a naval unit (a replacement for the caravel or a frigate unique from the English UU). Austria is probably the only other European civilization left considering, and would probably have a late-medeival or early-industrial UU. Adding the Incas and Mayas rounds out the Americas, though adding another Native American civ (maybe the Sioux) might add flavor. Africa probably could use the most expansion. Adding in civilizations like Ethiopia or Songhai would help in this matter. As for Asia, probably the most imporant addition would be a Southeast Asia civ like the Siamese. The Khmer (Cambodians) have also been mentioned. Either one would make a good addition.