Better army management.
The idea is very simple and isn't new at all, but I think it would fit the game graciously and add more realism to it: instead of make units, cities would product equipments, then you could take 1 pop of your city and equip that with the proper equipment, for the unit you want to create.
Yes, you saw that before: colonization.
I think this would be a fantastic feature, since Civ 5 apparently already implemented differences for the resources requirements for making units (they will use the resource stocked(?), or so I heard).
I also think it would fit perfectly the reality, without ruining the game experience: in real life, only countries that feels very threatened, or that have to pose as a major power, have huge standing armies. Most countries have a recruitment system, and can recruit more people in times of war. This also reflects the behavior of nations along the history as well: soldiers were mostly farmers, fishermen and the likes during peace times. When war comes, then people get on arms and go fight. Few historical empires were exceptions to that, and of course, that exception could be perfectly reflected in game, like ancient rome, who had standing armies across europe on borders of the empire.
So the system would be like you make equipment and stock these (with reduced upkeep, compared to a full equiped unit) until you equip it on a unit, wich comes from your pop. After all, it's weird that you pop grows while you make units, since the unit represent a large fighting force that comes from you pop.
When a equipment becomes obsolete, you can salvage it for a little hammer on making new equipment, while you can call your obsolete unit home and make them work. You can see that it matches reality completely, specially if you think about WW2 when the US army drafted a large force that was disbanded once the war was over. Those guys returned home to work, they didn't disappear.
Very simple, I think the only thing that would have a drastic change would be, of course, the AI, that would have to be all reworked.
This system would give a lot of military strategy choices for all nations, no matter the size or type of game they prefer.
That's my idea
Please share your thoughts about it.
Cheers.