I know it would never happen, but it would be really neat to get mini civ expansions based around a specific area or timeframe. Two specific examples I can think of:
Antiquity (timeframe) - Palmyra (Zenobia), Pontus or Parthia (Mithridates*), Illyria (Teuta), Gaul (Vercingetorix), Armenia (Tigranes II), Gothia (Alaric I), Carthage (Hannibal), Rome (Marcus Aurelius)
America (area) - Maya (Lady Six Sky), Cherokee (Dragging Canoe/John Ross), Apache (Geronimo), Salish (Seattle), Iroquois (Jigonhsasee), Muisca (Nemequene), Taino (Anacaona)
*Both countries have a ruler named Mithridates (two in Parthia's case) who is considered among, if not the greatest king(s) to rule over their respective kingdoms, so they're only interchangeable from the standpoint that you probably wouldn't see both together because their best leader choices share the same name