To me its just a great concept for an RTS to really advance through the ages and improve your civilisation. Sadly not a single game truly did it right yet. Many offered a fun experience for sure, but I never saw one that did every era justice, or assigned the appropriate building and unit styles to all civs.
It seems it would be better done with an expansion model, but with clear expansions. Just to get it all right.
The full game could just be europe and the middle east. Plenty of nations in there of course, but the build styles and unit styles would just be from those regions. All using accurate units for each era, the appropriate build style and likely being a bit more unique. Factions could share the same build style in some eras, but would not always have the same. Especially in earlier ages there would be more variation. Some factions could also change name through eras, aside from those that historically ceased to exist.
Basegame factions could be: (including build styles.)
-England/Brittain.
-Franks/France.
-Germany.
-Norway.
-Greece.
-Rome/Italy.
-Egypt.
-Persia.
-Babylon.
-Turkey.
-Carthage.
-Iberia/Spain.
This would pretty much cover it I think, they could also add a few that are quite similar and share some unique units with others just to pad it out further.
Like Byzantium for example, which could practicly be like Rome up untill the dark ages. Or Portugal, which could be quite like Spain in earlier ages.
In the stone age all civs would basicly be the same aside from their cultural bonuses they have through the entire game. Only the people would use different skintones and haircolors. Middle eastern cultures would have a darker skintone and black hair. Eastern european could be lighter skinned and have brown and black hair. North european could be blonde. (if scotland or ireland would be in the game, they could be red haired.)
They'd all get the same simple buildings. Units would be cavemen, spear throwers and villagers. It would pretty much just be about getting the good resources and getting the largest army. (population count should be huge.)
Next age could be classical.
Here there would be a great amount of differences, and the highest amount of differing build styles and units. Build styles would be: Egyptian, Middle eastern, Greek, barbarian, scandinavian.
Each faction would get a unique wonder to build, and all factions would get unique buildings too but they would be spread through the ages just like unique units. Sometimes replacing another, other times being an addition.
Typically in this age the scandinavian and barbarian build styles would have less buildings and theirs would be weaker, but they would be able to build and repair them faster. Their units typically would have a better offense than defense.
Armies would be quite different here.
You'd get the basics:
-Infantry.
-Spearman.
-Skirmisher.
-Archer.
-Light cavalry.
-Heavy Cavalry.
-Horse archer.
-Priest.
-Light ship.
-Heavy ship.
-Catapult.
Not all factions would get all of these units, and practicly all factions will have a unique replacement for something. Romans would obviously have the legionaire as their infantry replacement, which would get higher defense.
Heavy cavalry in this era would basicly be limited to middle eastern cathapracts, though some like the greeks their companion cavalry could fill the role too, but with less defence and a bit more mobility.
The horse archer would also be more middle eastern, though many other factions could have an alternative like spear throwers or chariot archers. But some might not get any ranged cavalry at all at this point, but could instead get an extra unit elsewhere. Like a ballista, war priest, extra infantry unit with unique ability, etc.
The general appearance of the units that are not unique could be tied to the buildstyle, so its always appropriate. Though in some cases a different one could be used due to the culture. Carthage for example may share some unit appearances with the greeks instead of middle eastern style.
Dark ages would be next.
Then medieval, where more factions start sharing a build style. Up next is gunpowder age, imperial age, napoleonic age, ww1, ww2, modern and future. Would pretty much cover it I think.
Overall I think units their strenghts and weaknesses should be relative.. An army of cavemen could still club some infantrymen to death when flanking them. It should be more the actual technology giving the advantage. And the cost. A plate armored knight would cost more than a modern infantry soldier. Tanks however would be very expensive but there wouldnt be much that early units could do against it. They would do light damage and would be able to destroy a tank, but it would cost them many units. Kind of like assaulting a fortress with melee infantry in most RTS games. Eventually you destroy it, but its costly.
Overall this might make the arms race more interesting and ensures those with a tech advantage still need to stay on their toes. Primitive stealth archers could devastate modern infantry when laying an ambush. The advantage is huge still, but you cant get careless.
The final epoch, the futuristic one, could be a little different than usual.. you could specialise in three different directions: Robotics, genetics and ideallic. Each with advantages and disadvantages, and each path offering a different unique unit for a civ. In addition to any they might have that apply for all three paths.
For robotics workers would be replaced with robots, and some units would be replaced with mechs, drones and other robotic stuff.
Genetics would be the most unethical thing since you'd work with mutations and such to improve your troops and workers, it would include some biological warfare as well.
Ideallic would be an ideal image of the future Kind of an ecological marvel with futuristic weaponry and such, but not taking extreme steps. Normal humans would operate it all. Mostly just improved versions of the modern day stuff. Still the typical idea of an army.
And for expansions they could focus on specific regions. Ideally it would be Asia and Americas, with Africa added as a smaller expansion or free update since it would add one build style while the others would add more than one.
Asia expansion could add China, Japan, Russia, Korea and India.
Americas expansion could add America, Aztecs, Inca and two native american tribes. Maybe Cherokee and Huron Confederacy.
Especially this one would be quite a challenging one to add due to different advancements in gunpowder and cavalry.
Its a what if scenario though.. the Aztecs and Inca could get firearms and cavalry, but later than others and just basic stuff. Before that they would get other advantages such as their infantry and ranged units being cheaper.