I would prefer for civ game vanilla version to always have these 14:
Egypt, Greece, Rome
Arabia, Persia, Sumer/Babylon
India, China, Mongolia
America, Russia, England, France, Germany
By the final version the game IMO "has to" contain also these four, in this exact form:
Japan, Turkey, Spain, Portugal
I maybe could survive without Aztec, Inca, Maya, Ethiopia, Korea, Netherlands, Carthago, Byzantium - but only if their historical/cultura/geographical regions got some really amazing replacements.
It is highly unlikely because each of those 8 was very impressive master in some sort of niche, but maaaaybe if you replaced them with Toltecs, Olmecs, some Andean empire, Swahilli, Vietnam, medieval Italy, Phoenicia, Kievan Rus/Bulgaria...
There are also "Celts" and "Vikings" who are IMO necessary but can be represented in at least three ways each (Scotland, Ireland, Gauls/Denmark, Norway, Sweden).
So, personally I want
Core Grand 18 (Egypt, Greece, Rome, Mesopotamia, Arabia, Persia, Turkey, India, China, Mongolia, Japan, Spain, Portugal, England, France, Germany, Russia, America)
Only With Amazing Replacements 8 (Aztec, Inca, Maya, Ethiopia, Korea, Netherlands, Carthago, Byzantium)
Duo of Many Faces ("Celts", "Vikings")
In the end, by the final version of the game, my dream civ contains also representation of
North American Natives
Latin America
a civ from Eastern Europe (between Istanbul, Berlin and Moscow)
Maghreb
West Africa
East Africa
Central/South Africa
Central Asia (other than Mongolia)
Continental South East Asia
Malayan Archipelago
but these can come in various forms.