Here is an idea for a completely different military system for Civ based on Sanguo (3 Kingdoms) games I've played.
Instead of building military units as is, we can split up the military into men + equipment. So you recruit/ conscript / draft your population to form military units, and arm them with era appropriate weapons which you have to build. So a swordsman unit of 5000 men would require recruiting 5000 men and also building 5000 swords. Upgrading units would simply mean building new weapons to replace the old ones. so in the modern era you build rifles or machine guns instead of swords of spears. There could also be a unit action called "training" where you train your units in the proficiency of a weapon system. You could always send untrained troops to combat, but they will be less effective. Combat experience, of course, will increase effectiveness beyond regular training.
This system could also allowed the giving / trading of arms to allies. So instead of directly involved in a war between 2 civs you could simply supply/sell arms to one of them. this sytem would not only make combat more realistic, but diplomacy more interesting as well
Instead of building military units as is, we can split up the military into men + equipment. So you recruit/ conscript / draft your population to form military units, and arm them with era appropriate weapons which you have to build. So a swordsman unit of 5000 men would require recruiting 5000 men and also building 5000 swords. Upgrading units would simply mean building new weapons to replace the old ones. so in the modern era you build rifles or machine guns instead of swords of spears. There could also be a unit action called "training" where you train your units in the proficiency of a weapon system. You could always send untrained troops to combat, but they will be less effective. Combat experience, of course, will increase effectiveness beyond regular training.
This system could also allowed the giving / trading of arms to allies. So instead of directly involved in a war between 2 civs you could simply supply/sell arms to one of them. this sytem would not only make combat more realistic, but diplomacy more interesting as well