Why does the game have to be like the unmodded one though? I think, rather than thinking in terms of balance, you should think in terms of fun.
Most civ players are history fans it seems, and I enjoy some historical accuracy in my civ games. Thats why i play it.
That said, I would like to see the earlier eras changed abit.
Avain you mentioned you didnt want to add things if they didnt have any cultural diversity. I think I may be able to provide a solution to this problem.
If you look at a popular mod thats out now, its the Stone age mod, and it has alot of good ideas.
Id be really epic, to start in a Stone Age, and have to build your civilization from the ground up. You would add more hunter-gatherer techs, a stone-axeman and tribal spearmen unit, as well as slingers and Atl-Atl(a mesoamerican term, but primitve javenlins were used for hunting and war), all the worker actions would be availible to research in this era. I'd maybe add some pre-galley/trireme ship here as well.
Ancient Age: This would begin where the Stone Age left off, basicly, its the Copper Age in the real world. City planning, masonry, organized religions, the Calender(should really be here rather than the Classical age). Basicly, its where the earliest known civilizations existed as, and the Mesoamericans and Incans later, with cities, roads, chariots, and later Horsemen. The standard Axeman, Swordsman, and Spearman, and archers would come into play here. Galleys would be availible once you researched a tech. Monarcy would have its start here.
Classical Age: The Bronze and Iron ages basicly. Not much could change here. But I would add perhaps, a heavy swordsman of some kind, as well as some sort of classical era cavalry, as a missing link between the Horseman and the Knight. You would also gain the Catapult. Early in this era, you would get the Trireme class of ship, perhaps with a naval warfare tech. I would maybe add a pristege "Siege Quinareme" unit for the first person to discover this tech. More forms of government would be interduced, direct representation for example.
Medival, Renaissse, you do a pretty good job with these already
.
Basicly, in terms of new units:
Stone age:
Alt-Atl(upgrades to archer)
Spearman(Upgrades to Ancient Spearman)
Axeman(Upgrades to Ancient axeman, or swordsman)
Slinger(Upgrades to archer)
Ancient Age:
None
Classical age:
Heavy Swordsmen(upgrades from Axemen or Swordsmen)
"Heavy Horsemen"(upgrades from Horseman)
Siege Trireme(first to discover naval warfare gets one)
Basicly this provides both historical accuracy, and more fun I think. If you want graphics for the units I mentioned above, they are quite availible.
. Look no further than the old graphics you used for some VD things, or even some BTS units that arent in the standard BtS game. You could use Extra for an example.
Also, why not add some more religions? Make them modular perhaps, and maybe have the option to choose which religions you have in your game. Be nice to have:
Nahualt/Mesoamericanism
Amun-Ra(Ancient Egyptian)
Aesir(Norse)
Hellenism(Greek/Roman religion).
Zorastrism
Look at the Extra mod for ideas for this as well. Again I'd ideally have a religion selector component for this. You could have the ones you wanted in game for variety or flavor, all selectable. If you wanted to have your own Ancient Medditarean flavored game, than you could have "Amun-Ra", Hellenism and maybe even Zorastrian religions in game for example.
Hey, maybe you could even add some more future buildings, and a "Future Religion", for the future age. rofl