I'd like to propose the ability to draw reserve armies from your cities.
This would only be available with a city that you occupy. Possibly even require one to occupy a city for a period of time before enabling.
When enabled, on the city page, there would be an option to purchase a reserve army. There is no 'geo' cost for drawing these armies, HOWEVER, upkeep expenses will still apply. Once drawn, the army is immediately deployed within that city, and can then be used by the player to defend that city.
Here's the catches though that would make this viable:
Reserve armies can not leave a city. They are permanently tied to the city they were drawn from.
Reserve armies lose 1-100 troops per turn in a fight, instead of 1-50.
You can only draw 1% of the cities population into a reserve army. (That's the max, you can draw less).
You can only draw one single reserve army per city.
You should also be able to disband a reserve army, allowing it to return to the city. In this case, you'd simply not have that army anymore, or the expenses associated with it.
To abandon a city, you would have to disband your reserve army first.
If your reserve army is defeated, you would NOT be able to draw another reserve army until someone else has occupied the city. Alternatively, if there is a time requirement in the first place to access the reserve army feature, require this time period to be met again before you can draw another reserve.
That may suffice, though there are a few more ideas that should be implemented if variable size cities are introduced.
Obviously, if city size is variable, drawing a reserve army from a city should diminish the cities population by the draw amount, and disbanding should increase the cities population by the number of troops left in the army.
Could be an interesting tactical bonus given for holding a given city successfully. I think the only thing to really figure out is what the optimal time requirement for a reserve draw would be. This is important, as I believe this is where proper balance would be determined. 1 day might suffice, and could be interesting in promoting battle resolution. Say you take NY. You're spread a little thin. Normally, whomever you took NY from would save up enough to retaliate in force, sometimes pretty quickly, sometimes a day or two later. With reserve armies available after 24 hours, this would provide a big incentive to retaliate asap, because you KNOW that if you wait too long, you'll not only have to wipe out the armies that took the city, but there could also be a 210000 man reserve army to deal with!