I'm on a kick where I'm solving all my problems using the 3e system (the fourier transform of a white noise function, for instance). One of these problems is mass combat.

My method is simple. Make a unit into a special instance of a character. In this manner, a unit can have classes, prestige classes, and feats. All of these could differentiate and personalize your military more. However, this causes a little more book-keeping. But, who would play Birthright if they truly disliked book-keeping?

Unit classes would be (essentially) the same as normal 3e classes. Here's a couple of examples:

Typical Levy
Human
Commoner
Level 1

Avg attributes (all 9)
Feats - Martial Weapon Proficiency (Pike), Endurance
Skills:
+4 Profession (Farmer), +4 Craft (Carpentry)


Typical Militia
Human
Commoner/Warrior
Level 1/1

Avg attributes (all 10)
Feats - Martial Weapon Proficiency (Pike), Alertness
Skills:
+5 Profession (Farmer), +1 Profession (Soldier), +4 Craft (Carpentry)


Veteran Militia
Human
Commoner/Warrior
Level 1/4

Attributes (Con 11, the rest 10)
Feats - Martial Weapon Proficiency (Pike), Alertness, Toughness
Skills:
+8 Profession (Farmer), +2 Profession (Soldier), +6 Craft (Carpentry)


Green Medium Infantry
Human
Fighter
Level 1

Attributes (Str 13, Con 12, rest 10)
Feats - Weapon Focus (Short Sword), Iron Will, Toughness
Skills:
+4 Profession (Soldier), +2 Craft (Armorer), +2 Craft (Weaponsmith)



These are just examples. The Iron Guard of Ghoere would have a few levels of fighter and a level or two of Iron Guard of Ghoere prestige class. Feats would work in a similar manner, as well as skills. The stats for the unit are not the exact stats for every character in the unit, but closer to an average. A unit would get a feat every third level, and class abilities, like bonus feats and sneak attack. Most magic using classes are ill-suited to units but not all of them. A unit of paladins healing itself in formation would be an interesting sight. They would have hit dice and saves by class. They would do damage by weapon type (criticals would work differently, though).

Certain aspects would be different, but a unit can function very similar to a character.

Hit points would work differently. A point would be a fraction of the units full health. When 10% of the hitpoints were lost, 10% of the unit would be out of commission. When a unit reaches a certain point (say 25, 50, and 75% hp gone) its combat effectiveness would decrease (possibly BAB, damage, AC). Healing would require the expenditure of time and experience points (replace veteran casualties with new recruits and train them).

Also, the class doesn't always determine the units abilities. A regular archer unit would have different feats and attributes than an equally experienced infantry unit, though they may have the same class. Different races would have their racial features, as well.

If anyone is interested in hearing more of my ideas on this matter, I'm willing to further share my thoughts. I am currently developing this idea.