Okay, I've finally had the flash of inspiration I've been waiting for. At last, a stat comparison system!
All models have a stat line, whose numbers are fixed as one would expect. Attacks, armor, and other special stuff adds die-rolls to these fixed numbers when actions compare them. Allow me to demonstrate with a completely hypothetical scenario with hypothetical placeholder stats.
Two identical cavemen, Uhg-A and Uhg-B, are battling (there can be only one Uhg!). Both wear the same hide armor and carry matching clubs. First, they have to actually engage each other in combat - this a function I'll work out later (Initiative and so forth). For argument's sake, Uhg-A strikes first. His basic Weapon Skill is 5, and his club offers a to-hit roll of +1D6. Uhg-B's basic Agility is 5, and his light hide armor allows a +1D6 roll for dodging (you can see where I'm going with this). Uhg-A connects, and rolls to wound Uhg-B. Uhg-A hits with Strength 5, and his wooden stick rolls a +1D6 when smacking people. Uhg-B wears some thick hide, adding +1D6 to his 5 of Toughness, and yeah you get the idea.
With this caveman example, let me stress a very important point. The system, as I envision it, makes absolutely no distinction between technologies, eras, or origins. For the purposes of balancing and model cost, it is the effect, and only the effect, that matters, not the explanation. If you make a unit of sling-swinging hobbits and say they do the same damage as riflemen, other players would be perfectly entitled to make fun of you, but as long as you pay the commiserate cost of rifle-like damage, you can say it's because they throw rocks at the speed of sound, and the system itself will not care one iota.
Back on point, the bonus rolls for "equipment" (explanations are meaningless) go up or down to represent various kinds of attacks or defenses, and also interact with the target's options. Examples: A buzzsaw would have a high Strength bonus for cutting through armor, but a very low Skill bonus because it's clumsy. A flamethrower would have relatively low Strength, but it's fan would downgrade the target's Agility bonus. A bulky suit of armor would have a high Toughness bonus, but add little to Agility. A very hard suit of armor would possibly degrade the weapon's Strength bonus or divide it's basic Strength, as it's designed to withstand direct blows. I'm picturing a large table of Weapon and Armor types and their respective interactions with each other. And of course, some weapons (like guns) will have their own basic Strength independent of the model that carries it.
It also bears mentioning that these bonus rolls will all be D6's, D3's, D8's, and so on. As there's no physical table or models, any random numbers needed could really be of any possible range, but I think for nerd-dom's sake if nothing else I'd like to adhere to the Platonic polyhedrals.
So I still need to think through and devise the whole squad-logic-initiative-morale thing, and a weapon design system, and armor types, and a terrain system. But at least I've got the basic stat-use-comparison matrix hammered into place. Although no real stats to plug into it yet. Anyway, there you go. Thoughts?