My overoptimistic wish list for future DLC would be:
Mesoamerica: Tarascan (and you could make an argument for Zapotec, Mixtecs ~Tlaxcala)
South East Asia: Thai & Mons (or Chams)
East Africa: Nubia & Somalia
West Africa: Kanem-Bornu & Songhai (West Africa more possibilities than those two)
South America: Chimú
East Asia: Tibet & Jurchen
North America: Mississippian, Iroquois & Algonquian