It's Prussia and France Obviously, Then UK 1871-80, then most likely the US 1870-80, but that's still up for debate. And then a 5th faction, full of "micro factions" where each unit is essentially a faction containing basically the most prominent units.
Since we're doing the weird Super Friends Team style thing for the 4th faction here is my suggestion:
Ottoman Line.
Mexican Line.
Russian Grenadiers.
Imperial Japanese Line.
Swedish Line.
Boxer Rebellion Chinese Soldier line.
Zulu Militia.
US Marines.
Italian Light Infantry.
Austrian Rifles.
Cossack Medium Cav.
American Dragoons.
Ottoman or Polish style Lancers.
This is a pretty good list, Harybo is working on the Swedish line, if anyone else wants to volunteer feel free