Actually, Aymar, is that really the case? RTR, for instance, had different skins for the same unit within the same faction based on the province it was recruited from. Since we now have the ability to define basically an infinite number of recruitment regions we could do this, if we have the time to do the skinning, and it is possible to do it without defining new units.

The problem as I see it is that we can't change the skin based on the season. So if that legion recruited in a northern area is in battle in North Africa in the middle of summer, they will still be wearing furs.