It is a long thread already, and I havent read it all, so just let me imagine myself in the shoes of Firaxis and put up 18 civs on the table:
Must be civs:
America: uh, well, the game is made here in this country, so this is number 1. We wont call it USA, though
it looks more ancient this way 
Okay, now lets start thinking about human history.
Oh, we all now what comes to mind first with history: Rome, Caesar and empire and Colossuem and all that popular image with that. Thus this is number 2.
Uh, but wait, Rome was preceded by Greece, all Europeans claim them as cultural ancestors. They are number 3.
What is more in ancient times? Yes, Cleopatra, and we need some hot chick anyways, and hey, the Pyramids, so we got Egypt and number 4.
Ancient times in a global view: yes, yes, China and India and we have number 5 and 6. And also big market to sell the game for
By the way, market, Japan must be included. Lucky thing is that they are civilization on their own right, too. So number 7 is checked.
So much for ancient times. Maybe later we get back to it.
We need plots for big nations, lets go by continents.
Europe, naturally, we concentrate on that, as there was so much history there we learned, about: Middle age, kings, wars, world wars
Naturally, England is inevitable, USA comes from there, so they are really a must. England is number 8.
France is next, Napoleon and those many Louise kings, Versaille and all. When we think of culture and civilization as European, we mean France they are number 9.
Big nations? Big market? World Wars? Germany, naturally. And Hitl.. err, anybody else that you can put there. Number 10.
Okay, but we can put in Stalin, cant we? We cant? Eh. Anyway, Russia is so big a land you cannot leave it out. Plus Cold War nostalgia.
Good thing we dont need to call them a Union. Number 11 is done.
Cold War is over, but we have a new exciting enemy, the arabs we must put them in. And some people tell me they are also civilized,
at least they have been around for a long time. Let Arabia have number 12.
This is all very evident, we can stop here, clean and safe.
What?
Africa? Uh, boy
What America? We have America, that is us. Wait
South? Ah, you mean some exotic stuff is needed?
OK, so we cant stop by 12, we must put in more
let it be 12 plus a half dozen then.
One for Africa, and one for South-America, then.
Hm, you checked our past games and forums and say Shaka is so iconic?
So barbarian and wild that he is loved? OK, Shaka then. Zulu, number 13.
What is there mentioned? Monty? Montgomerry? No? Montezuma? Ah, well.
Blood-sheding, sounds good. More Pyramids. Ok, so Aztecs. 14.
You say that Aztecs are not South? A pity. What the hell is there, then? Gold, more gold, Macchu Picchu, I see. Incas, then, is 15.
Likely civs:
Cant we stop here? We got 15.
Marketing? You say 15 looks shallow? 18 is more lucrative? Sounds like many? OK, Ok
Then lets check what we have so far:
Europe:
Rome, Greece
England, France, Germany, Russia.
Boy, how come we leave out so rich a market as those nordic countries?
Hm, but you cannot choose any of them, lets have them all as Vikings. Thats clever. 16, then.
Hm, you say that we will translate the game to languages and Spanish is a much spoken one? Heh, well, put that in then, so we already have 17, Spain.
You really want 18? Gimme a break
Just spin the globe and stop anywhere random
Australia? Come on, we already had South
I have better idea, let sell America twice, and give that last place to the Indians. No, not India, I mean
you know, that Sitting Bill guy? Bull? S...it.
Never mind, we can fold thus as 18. (Sioux)
Phew, never again we will make a civ game.
Whaaat? Expansions???
