Similar idea but not the same: city populations in Civ are discreet and small (the difference between 5 and 4 is very large). Civ games have always implied that city size isn't linear growth either, in Civ 5, a size 1 city was 1000 people, a size 2 was like 7000 and a size 3 was 21000 (The formula was 1000 * n^2.8). I think it would be interesting if all of these units subtracted from the growth of the city but didn't actually take away a population point. If a power value smaller than 2.8 was used for population scaling, this might make sense.
On the other hand, the wikipedia article for military formations states that a corps is a unit of 20-40k soldiers and a field army is 80+k soldiers. The level below corps is division, so if we assume that the base unit is a division, that would mean each unit is 10-15k soldiers. (This is a heck of a lot). Perhaps when you build these units, you should lose the equivalent of 10k people from your city. If we DID use a value of 2.8 (per Civ 5), that would mean that a size 3 city producing a unit would drop about half its population in doing so. This is kinda bad, so we'd be better off using a more reasonable unit composition, for example 1000 soldiers (although even this is a lot if you think about it). Realistically, the basic unit size should probably increase through eras. I just can't imagine an ancient city mustering a thousand troops so quickly (let alone a barbarian encampment with 1000 bandits running around).
The other thing I think would be cool would be if units could only heal in (or with line-of-sight to) your territory with zone-of-control breaking line of sight. When healing, the units would take from the growth of the city. This mechanic would make it much better to take as few casualties as possible in every situation because healing is expensive. If that's implemented though, it would make sense to take away the +50 hp for getting promotions (where are the extra soldiers coming from??) but at the same time, it would make sense to add +x HP from clearing barbarian encampments.
Changing the way military combat works could help with the above problem: let's assume that each unit has 10 000 soldiers. We also assume that healing takes citizens directly from the nearest city you own. We can also assume that unit health directly correlates to the % of 10 000 soldiers that remain. In earlier eras, maybe you produce a unit with 10 health, representing 1000 men. Maybe barbarians spawn with even less health than that.
I think unit combat strength is calculated where you get a -1 penalty for every 10 damage you've taken (something like that) but it would make more sense if it was something like -1% for every 1 damage taken.
In this scenario, you can possibly decide how many citizens you want to invest in units. Early barbarians will be a nuisance with pillaging your tiles but probably won't actually cause you any real trouble because they will probably spawn with 3-4 health (representing 300-400 bandits) while your state military will probably have 10 health. This would translate to 2.5-3x the combat strength.
A number of things would have to change if this was implemented: I believe damage is calculated deterministically where you plug in the combat strengths of each side and the damage dealt is output. That's gotta go because in this system you can't take 30 damage like it's nothing (that would be 3000 deaths). Instead, damage is relative. If one unit has 10 health and the other has 5, they're outnumbered 2:1, but the damage dealt to each side would have to be more like -1 and -4 or -5, resulting in 9 health on the stronger unit and maybe 1 health or a kill on the other side.
City strength would also have to be changed to account for unit size because we'd have to take into account the size of the garrison in the city. Perhaps it makes more sense if city bombardment is only possible if there is a military unit in the city. It consumes the movement for that unit and uses that unit's health to calculate damage.
Finally ships should use this system as well. Ships need two types of health: ship health and crew health. Ranged attacks deal ship damage and some crew damage. Melee attacks deal crew damage (the idea is that you pull aside the other ship and board them. That makes it a man-to-man fight rather than an attack against the ship).
The speed and attack strength of a ship is relative to the crew health (if your ship is only half manned, you won't be able to turn the sails, adjust ropes or fire as effectively, reducing attack damage and movement speed). Ship health affects strength and speed half as much as crew health does (so 50% crew health equals 50% the movement speed and attack damage whereas 50% ship health equals 75% movement speed and attack damage. 50% of each would be 25% movement and attack damage).
Ships are much larger than regular units so it's important to try to determine HOW many ships are in a unit. The number of ships is important because as we discussed earlier, you probably won't be making a full health unit when you produce them, especially early in the game. For the sake of it, let's assume that the number of sailors in any naval unit is 1000 to start and that each ship (I guess depending on the type of ship) is crewed by 100 of them. That means that each point of health represents 10 men and that there are a maximum of 10 ships per unit.
When you produce a naval unit, you have to fully crew it, so you produce 1 ship for every 100 men (if you only invest 500 men, you get 5 ships or half of a normal formation of them). When you attack another naval unit, you can attack as many ships as you have ships (so if you have 3 ships and the opponent has 4 ships, you can only attack at most 3 of them). While each "ship" in a naval unit carries it's own health, the whole unit moves as fast as the slowest ship involved. The damage of the other ships is not affected however. If the ship health drops to 0 on any of the ships in your unit, that ship sinks. 10% of the remaining crew are rescued by each remaining ship (so if you have 4 ships and 1 sinks, 30% of the crew from the sunken ship will join the crews of your remaining 3 ships). If the crew health drops to 0 during a melee attack (but the ship health is still positive) the victorious party captures the ship and adds it to their unit, splitting their crew equally among them. This is only possible in units below the ship limit (10 for a regular unit).
Ships can only be repaired in cities or harbors, crew health can regenerate in your territory.
So, even though this sounds a bit complicated, I think it's pretty straightforward: melee attacks risk your own crew's health in order to attack the opposing crew. The risk is higher but the reward is potentially great: salvaging the opponent's ship and adding it to your own navy. The safer alternative is ranged attacks: you limit your own chance of taking damage by attacking the enemy ship, but damage dealt this way is only half as effective at lowering the combat strength of the enemy and will either sink or badly damage a ship you could potentially capture.
