Cainehill, if early, middle, and late covered everything, then we wouldn't have early stoneage, late stoneage, new stoneage, ice age, copper age, bronze age, iron age, the Hellenistic era, the age of Rome, the Viking age, the middle ages, the late middle ages, the Cruisades, the Renaissance, the Reformation, the age of enlightenment, the industrial age, the silicon age, the space age, the modern age, 10,000 years of the future via "Orion's Belt" (neat site, try googling it)...
And I tend to disagree about it not being thematic to expand on a theme.
There's nothing wrong per se with having a mod that includes all nations of all ages, just like there's nothing wrong per se with a wargame that includes Roman legionairres fighting Civil War units and Warhammer units on an Ridley Scott's Alien-infested Mars. I think though that time-periods of time and the passage of time, as well as the "timing" of particular movements/actions within a given battle-is indeed a very deciding factor where war and strategy are concerned.
And thanks, Sandman, I appreciate that.