Emigration/ Immigration, corporations, and cultural intermingling are the mechanics in what I would want in a third expansion anyway. The three best new Civs to fit these themes, in my opinion, are Argentina, Italy, and Vietnam: Argentina- melting pot/immigration hub with the largest population of Europeans in South America as well as natives. Eva Peron Foundation as a corporation potentially? Italy- Medici bank corporation and bringing Italian culture to other civs in the world. Vietnam- Culturally defensive and retain their own cultural heritage throughout their history through outside influences.