When a city has completed its last item in the Building or Training Queue, you have turns of wasted production until you manage the problem.
I propose options to spend these turns on something rather than seeing them wasted. I'm calling it Fallback Production. You set your fallback production option in each city and when its last building is constructed or its last item is trained up, it picks up the Fallback option rather than sitting idle.
Naturally a bunch of this can be tied to new technology and building requirements and they solve various problems in a variety of ways.
Some interesting fallback option examples:
Convert gold (or whatever) to material (or whatever): The idle city is essentially putting out a mandate to traveling merchants, to its citizens, etc to sell its unneeded material to the city for the good of the empire.
Produce metal (leather/padded/enchanted) armor/weapons/items: The idle workshops in the city are commissioned by the empire to make armor items. Gold and metal are consumed in the process and every turn a few bits of armor are made. These are then equippable by the sovereign, champions and warriors of the empire.
-This one would solve one of my pet peeves with the game. The sovereign of the empire walks into a store and the peasant in charge of it demands hundreds of gold in exchange for a few bits of armor. Who does he think he is?!
It would take some balancing to keep this all fair and not really a way to get around normal unit upgrades but I think it would be a great option to have.