Boris Gudenuf
Deity
That is true. It could definitely work for an Endless Legend type of IP with made-up factions.
It would work fine for historical factions as well.
What everyone seems to miss here is that the current system of 'Limited Uniques' makes a basic, totally false assumption: that civilizations did not change historically In Response To Historical (read: In-Game) Situations. This is so demonstrably false as to be self-evident, yet that is exactly what the current system (and the so-far-revealed system for Civ VI) of Uniques assumes.
The bankruptcy of the system is indicated by the number of alternative uniques that the Mod community has come up with for the current Civ V civilizations. In addition to numerous, sometimes pretty obscure, new civilizations, by my count there are 3 new Americas, 4 different Egypts, 3 different Romes, 5 different Chinas, and 4 different Russias - not counting Russian 'predecessors' like Novgorod, Kiev, and the Grand Duchy of Muscovy.
In other words, the current system includes a tiny fraction of the real historical 'uniqueness' of the civilizations.
Instead, make a 'suite' of Uniques available, some peculiar to the geographical - technological - social policy - religion - whatever condition of the civ, and some absolutely peculiar to the civilization, based on its characteristics throughout its history. And those would 'become available' mostly only as you progressed through history/game, not all at the start of the game when the conditions are only potential, not real.
Example: England/Britain, which currently looks to have very geographically-specific Uniques in Civ VI - almost all of them are dependent for use on Britain building a multi-continent empire. This is perfectly appropriate for Victoria as the leader of Britain, but not at all appropriate for the entirety of British/English history.
Instead, give the starting English Player a set of choices: Uniques based on starting position geography.
Some of these would be 'generic' - better movement/food/fighting/resources from Jungle, Desert, Hills, Forest, etc.
Some would be English-Specific: better Resources from Stone, for instance (based on the prehistoric stone monuments, buildings, etc all over the British Isles)
Later, England would get choices based on the in-game situation:
English Uniques:
faster sea movement
Longbowmen
Billmen - an English Pike-Equivalent with more effect against Swordsmen
Magna Carta - more happiness from the nobility, faster adoption of advanced government types
Henry VIII - get one free complete Change or Adoption of Religion for Free
Race-Built - better warships in the Renaissance
Sea Dogs - legal pirates, basically
Royal Charter Companies - a way of extracting more money from Trade Routes and resources
The Royal Academy - an English-specific National Wonder for science
The '74' - a very cheap and efficient Ship-of-the-Line
Red Coat - musket infantry specialized for 'overseas' fighting
British Museum- more archaeological and tourist attractions
Battle Cruiser - a faster, but slightly weaker, Battleship that can be converted into an Aircraft Carrier later
These are just off-the-top-of-my-head examples, but I hope you get the point: ALL of the historical peculiarities of England/Britain can be included, not just a handful. Which ones you actually implement in a particular game will depend on how that particular game develops.
Start next to the Aztec, Zulus and Mongols? - plan on using lots of Longbowmen and Billmen to stay alive!
Have cities on three different continents? Develop Charter Companies and Race-Built ships, then '74s', Sea Dogs and Red Coats to protect it all.
In other words, with a well-designed and researched system of Uniques, I can play England a different way each time without feeling like I've wasted the unique abilities of that civilization.
In every case, the civilization will be uniquely England/Britain, but not necessarily exactly the same England/Britain in every game. In a game where, since the very first version, England has been able to build the Pyramids, isn't that kind of Alternate History what the game is al about?