Supply Lines should be an feature in my opinion built upon provinces whereas adjacent province could be suspectible to convoy raid without losing the entire province to an opposing empire but reduce the effectiveness and effciency of your soldiers, especially it should be capable of breaking down alltogether under intensive attacks, which would be maintained however by the amount of soldiers you have in that specific provine.
Supply Lines should be from your Capital Province towards whereever your soldiers are and from there they're vulnerable to major province invasions and raids or sea supply raids.
Perhaps what should be worked upon is an system of Conscription and Standing Army. Whereas your Society has an small, meager Standing Army consisting of professional soldiers with at your league the top-grade technology. From here on they can operate as defense/offensive/raid, ecetera.
They are not your elite however they can stand and gain grasp more battle experience than your Conscripts, furthermore fight more ferociously and standup agains't numerically superior opposition.
Conscripts are your peoples pulled away from your Societie's farms, shops, schools, ecetera and drawn into barracks and mildly trained. Society should have an slider system to determine whether how much training and equipment you put into your military for conscripts for those players that don't want an expensive large Professional Army but however want very armored and armed, especially large amounts of Conscripts to defeat opponents smaller or of an equal but perhaps technologically inferior.
But an really awesome system that should be implemented for all players, all Socieities is an large list of military skills and professiencies.
The idea is in your military screen their is two lists of abilities your population, ethnicly or something inherited,training wise or geographically, that gives soldiers unique or common techniques and abilities that increase their stats, capacity, ecetera.
Skills - This is an system of constant upkeep whether you're online or not and gained from wars and good, kept Standing Armies or an basis of yours.
Skills in this sense are those that improve and give your soldiers good and alienating traits. Before I go onto possible Skills, the Standing Army can only ulitize all or most skill at once and so much more than Conscripts that can only minimally use them in battle.
What Skills. Skills allow Soldiers to furthermore effectively use heavier armor or weaponry. An category of weapons such as stabbing, slashing, piercing or blunting.
Unlike Proficiences, Standing Army won't be using so many Skills and be capable all-at-once. However these Skills reach into Pioneering, Convential and Unconvential Warfare as well Psychological.
Proficences - This is an system of inheritency and fighting, also various other traits and Societie life gained abilities. This is an system where most players won't have the same trait as in my imagination there would be hundreds that come from so many different things from Society.
For example.
Mountaineering would be an trait earned and born with all of your X Societie's citizens and constantly aid your soldiers with maneuvering and operating in mountainish provinces and areas. Farming and Foraging, camping and hunting, fighting would be boosted alot in those areas.
Many other possible Proficiences could be obtained like, Mining, Herbalism, Foraging, Farm Machetting (can't think of an name, would boost cutting weapons and farming), Swamp pioneering and farming, ecetera. Many players would be so much more distinct with an system as this.