In my current campaign, there is about to be a war, and the players are likely to be on the fringe of it. 3.x had a nice 3rd party splatbook, Monte Cook's Cry Havoc, which I loved, but there doesn't appear to be anything like that available in the 5e world yet (if there is, please point me to it).
I've decided to homebrew some rules for armies (while working up larger rules for the war as a whole). For the first bit, I only need to be aware of how units of soldiers work mechanically, and I really don't want to treat them like 25-50 individuals on the battle map (makes it a lot of redundant rolling). So I've decided to treat them like swarms.
Units have two types: Skirmishers and Formation Fighters that work a little differently:
Skirmisher: This creature represents a group of soldiers fighting in a loose formation. This creature can enter spaces occupied by other creatures (unless that creature has Formation Fighting). This creature can move through any space that at least one member of the unit could move through.
Skirmisher Movement: This creature may move up to 10 feet without provoking attacks of Opportunity
Skirmisher Fighting: If this creature attacks an enemy that it occupies the same square as, it is treated as having advantage on attacks due to flanking.
Formation Fighter: This creature represents a group of soldiers fighting in a tight, coordinated formation. This creature can enter spaces occupied by other creatures (unless that creature has Formation Fighting). This creature can move through any space that that at least one member of the unit can move through.
Engage: Once during their turn, this creature can move 5ft at a free action towards an enemy they have attacked. This movement does not provoke attacks of opportunity.
(I feel like they should have some resistance to ranged attacks or something, but this is what I got so far).
Attacks: To abstract out the attacks, the unit basically gets an additional attack for every X HP the unit has left. These attacks should be at a higher attack bonus than the individual member of the unit would get, as they represent the overall group effort of attacking, but be fewer attacks than a group of individuals would get.
Defenses: Defenses should also be slightly higher than a typical unit of its type. I'm debating whether to give skirmishers resistance to ranged damage, to represent them being loosely packed, or perhaps giving these units some sort of "An individual single-target attack can only deal X damage to this unit" representing the difficulty of damaging more than a single guy in the unit.
Routing: I think I need to handle the unit "routing" in some way. Maybe if it gets below a certain threshold, it has to start making charisma saves or something and if it fails, it flees....or maybe breaks into a group of individuals that flee.
Joining a Unit: A character can join a unit by occupying the same space as them. They can chose to use their action to end their turn, and change their initiative to the same as the unit's. They move with the unit, but may use their action and bonus action normally. They may move within the unit itself, provided they do not exceed their total move for the turn. The character cannot be targeted individually, but attacks against the unit are randomized, and may hit the character (need to figure out how to do this effectively).
Special Attacks: Envisioning things like archers and spell casters, each unit will have some special attacks. I'm thinking things like archers being able to volley an area, dealing damage to everything in it. Spellcasters would just be casting spells, with some modifications (e.g. a group casting a charm spell could actually charm another unit, while a single target charm spell probably wouldn't work mechanically).
My question is largely what are the thoughts on balance for these things in 5e? The goal is to build a framework that can eventually work with large army battles that can more easily incorporate individuals and monsters. As it stands, a character can "join" a friendly unit, or be an individual on the battlefield. I also need to figure out how commanding an army would work, but that can be done later (thinking something like command units can make cha saves for the unit, and maybe provide some small bonus to damage). Is there some glaringly obvious problem or simpler solution for this?