I have been considering the supply line upkeep increase that is implemented as a difficulty modifier in the Warhammer campaigns. While the idea that supply lines stretching gives rise to increases in upkeep is logical, this should not be due to number of armies fielded, rather the distance to provinces belonging to your faction.
For example, playing as Lothern, if you intend to invade an offshore settlement, your upkeep should increase significantly, doubling or tripling, and your forces face attrition, but closer you get to your capital your upkeep should decrease, going down to near zero when garrisoned. And so the scheme is, closer you are to one of your settlements, the lower your upkeep, farther away you are, the higher your upkeep. Not only would this make it more challenging to expand your territory, thus increasing your late-game playability, but it would be more authentic and immersive then just a blunt increase in your upkeep. And, this way, garrisoning an army would be like disbanding your men, as is the case in a normal army. Think about it, every time we recruit a regiment, we are not paying the initial recruitment fee so that men can assemble in a rally point, but most likely for their gear and training and tents etc., so when you garrison your army, it would still appear as though your men are assembled, but they wouldn't be walking around in arms all day long without any danger, they would meddle with the population, making their own living in the meanwhile. There might also be a garrison or disband mode that allows you to reassemble your army without additional cost except for upkeep, if the former idea does not seem too authentic to you.
What do you think?