Or even simpler:
Have ONE generic civ-type: "human".
THEN, enable all those "European/American/etc" culture-resources the same way it goes now for the nation-resources, but with a much bigger number of varied requirements (so you won't build African culture in Tundra; the requirements could even be NEGATIVE: "no Tundra/Ice in the city vicinity"), including specific buildings and maybe even units (or civics), not just resources and city-surroundings.
THEN, like it is now, but with a MUCH more branched-out system: e.g. European=>Latin=>Roman=>Italian.
This will lead to MORE culture-choices, but LESS culture-numbers-present.
Well, probably - cause you'd need to meet a lot more requirements to get the final-step cultures.
So?
Oh, and for the GEM scenario:
Simply force-name any number of civs that have fixed locations and give them that exact mega-culture.
You can easily name them based on geography:
EastEast-European civ; NorthWest-Asian civ; SouthSouth-American civ; etc etc etc.
(YES, with at least TWO directional definers, where applicable - no need for TWO in Australia obviously.)
Oh, and you can make some LIMITING mechanism that would REMOVE the "EXTRA" civs - look at how it's done in Civ2:
You have a SET of civs with the same color, so only ONE of each set's civs can be present at any time.
Which again brings us to the mega-culture-civs being those exact sets.
It's up to you to decide the size of each set - could be one per continent or maybe two-three for the bigger ones.
Like:
Australia - ONE set, always.
Africa - ONE to FOUR sets, TWO being optimal ([N, S]; optional: [NW, NE, C, S]).
Europe - ONE to FIVE sets, TWO being optimal ([N, S] or [W, E]; optional: [N, S, C, W, E]).
Asia - ONE to FIVE sets, FOUR being optimal ([CN, CS, W, E]; optional: [N, CN, CS, W, E]).
Americas - TWO to FIVE sets, THREE being optimal ([N, C, S]; optional: [NW, NE, C, SW, SE]).
Total - SIX to TWENTY sets, TWELVE being optimal.
Reduces graphical problems for PCs a lot, yet still provides a very varied gameplay.
I hope you get my above idea.
It means you WON'T be having Romans AND Greeks as TWO civs (they obviously belong to the SAME SET) - but you CAN (probably) have both CULTURES in your (South?)EUROPEAN-set civ.
And the last word:
I do think it's better to FIRST focus on making the GEM-compatible civs/sets/whatever - and only THEN play-test them on random maps.
But it's up to you, of course.