For an expansion I think having civ specific bonuses to allies that they receive, either shared or not, might offer incentives to build stronger alliances. For example, if my Ally is America, I get the amenity of hit movies. If I'm an Ally of the Aztecs i gain bonuses when i work tiles next to lakes. Scythia as an ally provides 2 free horse resources. Age of Empires has this mechanic and it's pretty fun. It would encourage you to honor your alliances and it would make certain civs more important on the world stage during the eras their bonuses happened to be most useful.