I'm in the process of brewing up my own mass combat system and I was wondering if I could get some feedback from anyone. It's based in part on what is freely available online of the D&D Miniatures Handbook en Malhavoc's Cry Havoc, neither of which I own.
Here's what I've come up with so far:

Units: First, a unit must contain only one kind of creature, except for the unit's commander, which may be a of a different type. Having commander in a unit is optional. Second, every unit regardless of type has a maximum size of 25 by 25 feet. A unit always is 25 foot wide, regardless of how many creatures it contains and what size those figures are. Units come in two types: formed and unformed. Generally, only those creatures at the outside edges (usually the front rank) of a unit can fight. A unit has an attack bonus, an Armor Class, a speed, and all other statistics that an individual creature has. Each of a unit's statistics is the average of that statistic over all the creatures in the unit. A unit can take almost any action in combat that a character can, and resolution of its actions occurs in much the same way. A single figure on a battle mat represents a unit in a battle just as it does a character in an adventure, except that a unit figure represents multiple actual creatures. A unit on the battlefield can attack other units, or it can attack individuals. Conversely, both individuals and units can attack a unit.

Formed Units: A formed unit of medium creatures contains up to 5 ranks of 5 creatures each. If the creatures are small, the unit can contain up to 6 ranks of 6 creatures, and if the creatures are large, the unit can consist of up to 3 ranks of 3 creatures. A formed unit has a clearly defined front, sides, and back that affect both combat and movement. Formed units can move in only two directions: straight ahead and straight back. Turning requires a pivot, which has a movement cost of 25 feet per 45 degree turn. A formed unit attacking to its front gets a +2 attack bonus. If attacked from the front, it gets a +2 AC bonus.

Unformed Units: An unformed unit of large creatures can contain no more than 2 ranks of 2 creatures, medium-size creatures have a maximum of 3 ranks of 3 creatures per rank, and small creatures have a maximum of 4 ranks of 4 creatures. An unformed unit has no facing. It moves and fights equally well in any direction. Because unformed units have no facing, they can move in any direction. In real-world terms, they function as both skirmishers and missile units.

Ranks: You can get significant bonuses on your morale checks by having deep units; that is, units with lots of full ranks. A full rank is a rank that has the maximum number of creatures in it. Each full rank behind the first adds +2 to the save. A full-size unit of medium creatures (five full ranks) gets +8 on its morale save, before commander effects.

Command: A unit is under command if it has a clear line of sight to its commander from anywhere on the battlefield or if it's within one 25-foot space of its commander. Commanders, of course, are always under command. A unit adds the Commander rating of the highest commander commanding it to its morale saves. A unit that is out of command attempts to attack whichever enemy happens to be closest to it. An out of command unit can do anything a unit under command can do. The only difference is that the out of command unit has a speed of 5 feet instead of its normal speed. A lone commander can always add its Commander rating to its own morale saves, even when it's stunned or otherwise unable to command other creatures. A commander can use its own Commander rating when it makes a morale save, or that of another commander, but not both.

Commander rating: A creature designated as a commander starts with a base Commander rating of 1, +1 for each feature listed under Higher Commander Rating and –1 for each feature listed under Lower Commander Rating. A commander's minimum Commander rating is 0.
  • Lower Commander Rating
    • Chaotic alignment
    • Int 8 or less
    • Wis 8 or less
    • Cha 8 or less
    • Rogue, sorcerer, or wizard
    • Character level 1st or 2nd

Battle Rounds: Unit combat takes place in 15-second battle rounds rather than the six-second rounds used for combat between individuals.

Initiative: Each round, you roll initiative for your whole army. You don't roll for each individual unit. To determine initiative, roll a d20 and add your commander-in-chief's Commander rating. Each round, players take turns each activating two units at a time until all the players have activated all their units. When a unit is activated, it can: move its speed and then attack, or attack and then move; move twice; stand still and attack multiple times (if capable of doing so); or charge and attack.

Targeting: A unit must usually shoot, cast a spell at, or charge the nearest enemy. You can't just pick off the enemy commander. Ranged attacks must target the nearest enemy regardless of whether that enemy has cover, is in melee, or is otherwise not an optimal target.

Attacks of Opportunity: There's no 5-foot step (or 25-foot step, for that matter), no limit to the number of attacks of opportunity per round, and no attacks of opportunity for reach attacks.

Damage: All damage and hit points are measured in multiples of 5. Hit points are rounded up to the nearest multiple of 5. Damage is averaged as well as rounded to the nearest factor of 5. Calculate average damage by adding the minimum and maximum damage together and dividing by 2. An ogre, for example, has 30 hit points (29, rounded up) and deals 15 points of damage (average of 2d8 + 7 equals (2 + 2 × 8) / 2 + 7 = 16, rounded to nearest factor of 5) with its greatclub.

Saves: A unit uses the average level of the creatures in it as its saving throw bonus.

Morale: Whenever a unit takes damage, it must make a morale save against DC 20. To make a morale save for a unit, roll a d20 and add the units saving throw bonus and, if the unit is under command, the Commander rating of the highest Commander commanding the unit. Success means it can keep fighting; failure means it runs away unless its commander brings it back under control. Commanders can try to rally units, to stop them from running. When a unit fails a morale save on another unit's turn, it runs away immediately. It then runs again on its own turn if it fails to rally.

Movement: Each unit has a speed measured in feet, equal to the speed of the slowest creature in the unit. A unit can run, but only if ordered to do so. Unless it is under orders to the contrary, a unit must move toward the closest enemy it can see. A unit in melee contact with an enemy unit (that is, within 25 feet of it) cannot move away unless ordered to do so. If so ordered, it breaks melee contact, usually provoking an attack of opportunity from the enemy unit.

Grids and Miniatures: Unit combat takes place on a square grid with one square equal to 25 feet. Simply place markers or figures on the grid to indicate the positions of units.

Cover: Cover grants a flat +4 bonus to Armor Class regardless of the number of terrain features or units that provide cover to the target.

Base Line and Rendezvous Point: Each force involved in a battle treats one edge of the battlefield as its base line—usually the edge from which it entered the battle. A unit that fails a morale check often retreats toward its base line. The troops in any unit have orders to go to a prearranged place on the battlefield when they don't know what else to do. Troops that find themselves without orders and without line of sight to any enemy units can move toward this rendezvous point.

So, what do you think? In particular: what do you think is missing?