Lets suppose that units generated by the mobilization of minor ally nations can visually be distinguished rather than simply merge into the armies of the controlling power.
This might just mean, for example, a yellow stripe on a French infantry to mark it as Belgian. With all the new pieces available if you contact WOTC this becomes practical.
What differences might these units have?
+1 in combat when defending their homeland; -1 elsewhere, or if their homeland is under enemy occupation?
e.g. - If Poland is under German occupation, will Polish units in the Russian army be less effective fighting for Russia, unless it means fighting over Poland itself?
option to place a new unit in the respective homeland each turn (with a sensible total limit - probably the original starting number; i.e. you can never have more than 3 & 1 Belgian inf and art).
One might extend the principle to colonies, e.g. marking some UK units as Canadian, South African, Indian, Anzac etc.
Would separate Bavarian units be representative of German political divisions?
Polish corps in the Russian army?
Austria-Hungary might have a real problem…