I'm thinking about letting my players control armies in the future and am trying to figure out a rule system that is tactical without being completely separate from the normal rules for combat. So players can attack an army without having to target a specific individual. I've not looked at rules from any other threads or books (I'm too cheap to buy them anyway), and I haven't really tested this out apart from rolling a few dice. Keep in mind this assumes that each army only has 10 melee users so it'll need to be tweaked to accommodate bigger armies. Some other things I want to make rules for later are: forming new armies, replacing fallen units, commanders, feats/skills for armies and commanders to learn, bonuses for tactics such as flanking or formation, cover and terrain, morale, costs of upkeep, and post battle loot and experience.
So when creating an army, take the health of each person plus their constitution modifier and add the total to create the health pool of the army. When the army takes so much damage (to be determined), one unit dies and takes any kind of bonus they contributed with it. Every unit in an army makes all of their normal attacks at once, I haven't figured out how to account for actions yet. The AC of each unit is used to create damage reduction so combat is less swingy while buying equipment still holds value. I've not quite figured out how to balance this but it should be taken from the average so more units doesn't necessarily make you stronger. I think if every unit has an AC of 18 they should be taking no more than half damage.
-Damage reduction is less effective against arrows and spells without the use of shields.
-Commanders can't be damaged by attacks from enemy armies. Players can still target them. Commanders are the big boss that adds flavor to each army through whatever special skills they have so they should be the last to fall, which gives them a chance to become a reoccurring character if they can escape (think Shadow of Mordor).
Any input on this is appreciated. I think I've got a good idea here but it just needs to be fleshed out more and balanced. If this gets a lot of discussion I may make a google doc to let anyone contribute ideas for feats and such.