well, I wrote about dividing nations into cultural groups a long time ago, in a different ideas thread.
Here are the groupings I came up with, 29 groups of 3 tribes each for a total of 87 nations, with a total limit of 29 nations in play at the start of each game. Nations with 1 in front of them would have 3 great leaders available, 3 unique units, 3 unique buildings and 3 own wonders, number 2 nations 2 of each and third ranked nations 1 of each only- this due to their significance in history.
This totals up: 87 nations, 174 leaders, 174 unique units, 174 unique buildings, 174 unique wonders, not too shabby.
Currently it took few years to release 43 civilisations with 43 leaders and 86 unique units. Best mod team need something like few weeks to release high quality civilisation (without animated leaderscreen and voice

). Good luck with creating for 2x number of nations, 4x number of (animated, right?) leaders and 6x number of unique interesting features (with 3d models and so on, right?). One of the most awesome features of modding experience is that it allows on better understanding how games and their design works. Your demand is completely and entirely unrealistic, and even if it would be done I am pretty sure players would prefer to have few times less features - but these features being truly truly high quality and unique. No, resources of designers are not infinite. Firaxis, big rich professional team, makes one leaderscreen in 4 weeks and I am currently observing the amateur process of creation of animated leaderscreen which is such a sophisticated process that it takes weeks and only ONE single person in our communiy [Ekmek] has the ability to do that.
And if you wanna to sacrifice animated leaderscreen and truly unique civilisations mechanics introduced by Civ5, why do we need all these boring low quality civilisations? Because doing all of that in time frame shorter than 5 years alone would require great sacrifice of quality.
There is a mod project in C&C in which I am helping, which aims at adding new unique feature to each original civ. HIGH QUALITY feature, not some crap as 'this unit will have recycled art and reused trait' - high quality art, 3d models and unique abilities. It has been taking something like half of a year and is close to finish but not completed yet.
Realism, my friend, realism.
Here it is, think of it what you will:
Groups:
ONE: 1 ROME 2 ITALY 3 VENICE
TWO: 1 GREECE 2 MACEDONIA 3 MINOANS
You have just made 'ancient scenario' with Rome, Greece, Macedonia and Minoans impossible
FOUR: 1. INDIA 2 MAURYANS 3 MUGHAL(or Harsha, or other Indian empire of the past)
Currently More Civs team has released India Civilisations Pack with India, Maurya, Mughals, Chola and Maratha. Creating this one took them a lot of time but the result is awesome - do I have to mention that nobody has a problem with all these civs being in the same game and many people love playing 'full India' games? You restrict mods and players' freedom, and these guys don't like that. The first thing to appear for this game would be a mod which removes all group restrictions.
FIVE: 1.EGYPT 2 SIAM 3 SHAWNEE- chose Shawnee here, instead of Cherokee, or other native north American tribe because of their great chief-Tecumseh.
Wait. What Egypt, Siam and Shawnee have in common? Oh and I wanna to play with Egypt and Siam in the same game
SIX 1. GREAT BRITAIN 2 ENGLAND 3 TIBET
Who in Eath needs both UK and England in civ game at all, instead of for example England and exotic interesting nation as Srivivaya Empire?
NINE: 1. RUSSIA 2 KIYEVAN RUS 3 BULGARIA- I know, all of them might be in together, but is it realistic?
You have just killed my medieval scenario with Kievan Rus and Bulgaria.
TEN: 1 BYZANTIUM 2 PRUSSIA 3 IROQUOIS
People wanna to have Byzantium, Prussia and Iroquis in the same game (what these all nations have in common???)
TWELVE: 1 TURKEY(Ottomans) 2 ZULU 3 POLYNESIA.
People wanna to have Ottomans, Zulu and Polynesia in the same game. You destroy their freedom.
THIRTEEN: 1 BABYLON 2 SUMERIA 3 SIOUX- Babylon and sumeria together, reason? more cities available to each( sharing of city names)
You have just killed Mesopotamia scenario. Oh, and cities are absolutely no problem.
FOURTEEN: 1 AUSTRIA or AUSTRO-HUNGARY 2 HUNGARY 3 AVARS- the barbarian Avars inhabited the lands of modern day Hungary(among others)- and I know, Austria and Hungary together? well, they were a single nation for quite a while
You have just killed medieval scenario with Austria and Hungary/Avars, and annoyed the entire Hungarian nation which fought for centuries to release from Austrian regime and now falls into the same 'cultural group'
FIFTEEN: 1 USA 2 PHOENICIA 3 KHAZARS- us never met any of these, different eras
People wanna to have USA, Phoenicia and Khazars in the same game. And I still don't understand whether your division tries to group civs with different or similar history. For me it seems that it randomly mixes everything
SIXTEEN: 1 NETHERLANDS 2 HITTITES 3 BELGIUM
You have just murdered European scenario with both Netherlands and Belgium.
SEVENTEEN: 1 VIKINGS 2 DENMARK 3 NORWAY- I know, Norway and Denmark, why? common ancestry with Vikings
Who in Earth needs all these three Viking nations in the same Civ game, instead of Viking Naval Superpower, Kilwa Loose Trading Confederation and Golden Orde Steppe Empire?
EIGHTEEN: 1 ETHIOPIA 2 SCOTLAND 3 NUBIA
Oh, you have just killed my African mod with both Nubia and Ethiopia.
NINETEEN: 1 MOROCCO 2 KOREA 3 SONGHAI- korea could be number one here, not sure
The same thing with Morocco and Songhai - people wanna to have both these nations.
TWENTY: 1 MONGOLIA 2 TATARS 3 HUNS
Asian scenario...
TWENTY ONE: 1 PERSIA 2 IRAN 3 ? perhaps one of: Medeans, Parthia or Elam
Asian/ancient scenario...
TWENTY FOUR: 1 INCA 2 ISRAEL 3 CONGO
Very popular nations. Congratulations, you have just forbidden players from playing all of them in the single game
TWENTY SIX: 1 MAYA 2 POLAND 3 MALI
Again, very popular nations. I am Polish and I love my civ being in game. Also I am the fan of Mali and Maya. Oh, and these nations have absolutely nothing in common.
there you go, ancestry or descendants included in same group, non historical contact also inclusive, no cultural or historical connection allowed in same group, well, with exceptions.

Read it slowly. To sum up, you randomly mixed all civilisations - some of them are ancestors, some have merely cultural links, some have 'historical connection', some are forbidden from having cultural links, some are randomly mixed based on 'non historical contact' (???), and there are also exceptions! You have just basically proven that this entire system cannot be applied universally to the game, it has some exceptions and ridiculous rules,
is counterintuitive and apparently understood and liked only by you
Furthermore on this idea, number one nations in their group would have a higher chance of being chosen as your opponent then the second or third seed, ratio: rank 1:43%, 2: 33%, 3rd:23%.
nations with 3 leaders- Leader number one: 4 traits, second leader: 3 traits, 3rd with 2 traits only.
nations with 2 leaders(ranked second in their respective group), leader 1-3 traits, leader 2- 2 traits
nations with only 1 leader- 2 traits for the leader, also these leaders would be selected upon the same system as the nations: top leader with 43% chance of being chosen by AI, second with 33% and so on.
Ridiculous complications, hell for modders and scenarios, great potential of programming bugs and glitches, annoying fanbase by supporting some civs while penalizing others.
Again, PLAYERS WANT MORE TRIBES INCLUDED! this plan allows nations like for example: ancient Persia and contemporary Iran to be included in the game, doing it any other way, to me, is anti historical.
Players want more diverse set of civs and better gameplay. They don't wanna Lesser Poland, Greater Poland, Masovia, Pomerania, Silesia, Roztocze, Belarus, the Empire of Lublin, Polish People's Republic, Polish - Lithuanian Commonwealth and pre - Polish ancient Slavic culture of Biskupin being in the same game. They wanna Persia, Kongo, Vietnam, Cahokia and Tibet. Each of these civs - as diverse and interesting as possible. And they want simple yet deep mechanisms, not ridiculously convoluted rankings, divisions, groupings, percentage chances, unclear categories and quantity over quality.
And above all, people wanna fun and freedom while playing 'empire simulator' - your division takes part of this freedom away. Oh, and believe me, mod community would hate this entire system
