I think the point was missed quite some time ago in civ selection. Somewhere along the line, the concept of a civilization became confused with that of the nation-state. The US is a sticky one, because it is a recent development, yet one which has had a near-unprecedented impact on the world. As such I think it's worth considering it as an exception.
The game begins several millennia BC, and each civ is expected to develop things like writing and bronze working etc and create great wonders of the world. We need to reach back into human history to find the civilizations that are still with us today and gave rise to the myriad nation-states we have in our modern world. I'm thinking the list should run something like this:
EUROPEAN
Greeks
Romans
Celts (no Irish? especially with missionaries now being part of the game...)
Slavs
Germans
if you really want to get picky you can deliniate
English (or British, though I'd consider Scots and Welsh 'Celts')
French
Spanish
even though all are the product of germanic invasions of celtic peoples under long-term roman rule
*Americans (USA) - more European than 'American' in the sense of indigenous civilizations of the Americas
AFRICAN
Egypt
Zulu
Benin (west Africa)
Ethopia
AMERICAN
Aztec
Inca
Olmec
Like Africa, while there were other civilizations (eg - distinct art, language, way of life) nothing of the scope that is typical of a Civ civilization (great monument building, world affecting, leaving behind a literature, conquering etc etc etc)
ASIAN
Mesopotamian
Persian
Arab
Indian
Chinese (through all of its manifestations...again, we do cover 6000 years here)
Mongolian
Japanese
Cambodian
that makes for 24 if all are included...you could even have an open slot (to make 25) so borderline civs such as Scandinavians or Phoenicians could be added...