For those that are not aware, Mantic will release soon a historical version of Kings of War.
(was initially planned to be released in August, but seems to have been slightly delayed, in any case it's still expected later this year for now)

It will more or less be the same core rules than for the Fantasy version, but the army composition will be handled completly differently, and there will of course be new or variant special rules to cover appropriate units.

The game will cover the following eras :

Antiquity. Pre 5th century AD

Early Middle Ages. The 5th to 10th centuries

High Middle Ages. The 11th, 12th and 13th centuries

Late Middle Ages. The 14th and 15th centuries
One of the biggest difference with the original KoW, is that instead of army lists, there will be a core list of generic units, with things like "spearmen", "archers", "knight", and so on ... and when you build an army you chose a theme that will describe your army nation and era, and it's the theme that will tell you which units from the core list you can use and any additionnal restrictions, special rules or other options you get access to.

For exemple I suppose that egyptians and celts will certainly get access to chariot units, but maybe the celts won't have the option for bows but get throwing weapon instead (pure theory, I'm just giving an exemple of what might be), or the egyptians might get access to different options like elite veteran charioteers, and of course I suppose that other themes like Romans or medieval France won't get any chariot at all.

Here's the list of all the themes :

Barbarians
Carthaginians
Chinese
Egyptians
Greeks
Huns
Indian
Macedonians & Successors
Persians
Romans
Spartans
Scythians
Thracians
Anglo-Saxons
Byzantines
Franks
Normans
Polish
Slavs
Vikings
Crusaders
Japanese
Mongols
Ottomans
English
Aztecs
Conquistadors
French
Holy Roman Empire
Scottish
There is also a section that will add some mythological options for some (all ?) themes, but it's supposed to be completly optionnal, the game is after all supposed to be for historical armies first, for the most fantasy options the normal Kings of War is already available.

And with the core rules being more or less the same, I suppose that playing pure historical armies against fantasy ones should be possible, for those that would want to play roman legions against hordes of undead, or an army of satyr and centaurs (the Herd army list) against greek hoplites.