The way I see it now every unit would have individual health (how much damage a stack takes before an unit is killed), stack size (how many fit in a single stack), offensive power for different targets and armor rating.
I would use random number generators instead of rolls for this. As I see it, if 1 infantryman has an anti-infantry attack rating of 1-3, a stack of 10 infantry would have do 10-30 damage. Individual shot accuracy wouldn't be counted and the attack rating stands for the combined statistic effectiveness of the weapon and skill of the unit. Basically a round represents multiple shots, but only one number is generated, which represents the total damage done by all the shots that hit. So both a sniper and a minigun would have equal effectiveness, because a minigun would do a lot of shots, of which not a lot hit and don't deal much damage, while the sniper does less shots, of which most hit and do a lot of damage. Rounds would represent a lot of time.
There could be different armour and damage classes. Light (infantry), medium (power armor, light vehicles) and heavy (tanks). Rocket launchers wouldn't do a lot of damage, but be able to penetrate tank armor, while a LMG would be deadly, but be 100% powerless against medium-heavy armor. Infantry have a lot of HP, tanks have a lot of armor. Commanders could be attached to stacks. They could have a turn separate from the stack, but when the stack is attacked, the commander dies last. That way commanders would be as mortal as simple infantrymen, but have larger chances of survival.
That way I could count 1 commander, 1 stack of 5 tanks and 2 stacks of 50 infantry as 3 units, represent a lot of casualties and have hard counters. Thank you for the link. It was very helpful.
Hmmm...now about the setting. When I thought up the rules, I was thinking about the Imperial Guard of WH40k, but that's unoriginal and I'm not that good with WH40k fluff. I only know about it from the Dawn of War games and viewing tabletop figures. I know what weapons make sense for what units and wouldn't screw up too much, but I don't know anything interesting.
The other option would be a typical soldiers vs alien invaders scenario, but that's overdone.
We could also have a realistic modern/WW2 war scenario, but personally, I think that's boring as hell.
Also, if I'm making this, I'm making it modern-futuristic. I want to use the "rifles don't damage tanks" hard counters.