The concept of a nation is probably as old as humankind. The concept of a nation-state is a few centuries old. A nation is simply a group of people who are ethnically and linguistically related; classic example is the Hellenes understood themselves to be a nation, even if their political allegiance was to Athens, Sparta, Corinth, etc.
Of course there was always the dichotomy of "us" versus "them", in which the "us" was usually defined in a way that was politically convenient. You can retrofit the "nation" concept onto that, but until the emergence of nationalism that is an ill-defined concept.
A civ unlock is just a next-age civ which is in some way related to the current civ. If you look at the current civs, the only civs that are real precursors to their unlocks are the Chinese ones.I don't know guys, for me it is quiet simple. There are 3 possible unlocks: Gameplay (for players who do not care a lot about historic accuarcy); Leader (for players who identify with Leaders mostly, not the Civs) and a Civ Unlock (which, at least according to my interpreation, means exactly what I have stated before: "The modern civ should identify with its precursor state"). Obviously it is very difficult to implement the latter, but that's at least what I would expect, and I guess I'm not the only one (see the various discussions in the Content Spreadsheet Thread).
Still don't understand how you would define a Civ Unlock then?