I have thought up a few modifiers for general variables based around the game's concepts and D20 systems, that are of course suggestions and fit to alteration and be discussed.
Terrain [Offensive Penalties]:
Plains: Easily accessible and transversable, optimal fighting ground for invasion force. +1
Woods: Some trees and vegetation. -0
Hills: Uneven gradients, difficult to move through. -2
Forest: Thick trees, rough ground, massive vegetation, canopy blocks sunlight. -3
Ice Flats: Very cold, harsh weather, exposed and barren. -3
Mountains: Extreme steepness, difficult terrain, lack of resources. -4, -5 for "Heavier" Troops
Desert: Extremely hot, arid, few resources. -6, -8 for "Heavier" Troops
Training [General Combat Modifiers]:
None: Drafted peasants. -3
Basic Training: General "How to stab things and not get killed immediately" teaching. Local militia level. +0
Standard Training: Adequate training that a regular, nondescript army would have. +2
Efficient Training: Soldiers that know how to fight and adapt in a variety of situations. The highest level one could hope for in a standard army. +4
Specialized Training: A handful of elite soldiers that can crush enemies whom outnumber them. On-par with warriors such as the Spartans and Elite Knights. +6
War Machine: An extraordinarily small amount of warriors that constitute maybe 100 per 100,000 normal soldiers, if even that much. They've been bred by birth for fighting, whether through slavery or inherited status. There's not enough of them to create a full force, so they have to be specialized for tasks such as guarding the leader(s) of state or a taskforce to complete duties no other soldier could. +8
Training also effects overall Discipline. More Discipline, means that troops will be less likely to Retreat, which could be based on percentages. For example, troops with No Training would run away after losing a mere 10-15% of their total force, for standard soldiers it could be 40%-60%, and War Machines will fight down to the last soldier.
Political/Ideological/Culture/etc. [Offensive Bonuses]:
Pacifism/Peaceful: Rejection of all fighting. Why would they do this in the first place? -4, if an offense can be sustained at all.
Isolationist: The desire to tend to their own problems rather than that of others. -4 if offense can not be proven to the populace to be in response to a direct attack or threat to the nation.
No Standard War Ideology: War is present, but the people of the nation don't have a standard outlook on it as a whole. +0 in general, varies on war.
Interventionist: The people feel that they need to fight in external wars for the good of their country. +2
Patriotism/Jingoist: War is a means to protect the homeland and promote the prosperity of the country. +4
Warrior Tradition: War is not just an abstract concept, it's an important part of everyday life, often embedded culturally and/or religiously. +5
Size also has to be accounted for, as an army is going to have a much easier time fighting another army if it's opposition is 1/2 or 1/3 it's size. Then again, larger forces would be more susceptible to Terrain, Supply and Morale penalties than smaller, more tightly-knit outfits.
Material is also important for combat in determining the effectiveness of a force's equipment. I imagine it works something like:
Nothing < Wood < Stone < Metal
The Quality of Equipment is also important, as finely, professionally crafted Wood weapons could very well manage to penetrate bad, amateurishly produced Stone armor.
A central part of combat is going to be the dynamics of the different types of forces, which currently look like Infantry, Ranged/Archery and Artillery.