Lets just add shit pathing for infantry units^^
1. Give them a direct debuff. We could call it whatever we want
Not enough supply in the area, enemy shots will hit somebody, enemys get a moraleboost when being outnumbered, logistics or the chaincommmand get disturbed, whatever. But that would be pretty radical, and somewhat lazy.
2. Better options to deal with blobs/ greater risks with blobbing. This could be mortars who get cheaper flaming barrages, or generall bigger (and maybe weaker) aoe. Or better MGs. The biggest problem here, is to find the fine balance, where the units are specifically better against big blobs, not against everything.
3. More incentives to spread out. Mostly through mapdesign, or broader mechanic changes. For example maybe you get bonus ressources when you have a unit in a territory (enemy or friendly), but more than one in the same area gives no advantage. It could be a supply thing, lets say the upkeep gets reduced for 1 unit per sector?
I could see something like that working; just give the MG a suppression bonus vs. grouped units.
Basically, give each MG an "AOE" (invisible) that centers around where they are firing (easy to figure this out via bullet path). For every unit inside that AOE Oval (centered on the firing target, but stretched out behind and infront of the target), suppression increases by X.
As a result, if you try to run a blob of infantry through a choke-point (any of those "bridges" on semois), you'd end up with a huge number of units in that AoE-Oval, and as a result, suppression would be massive (I would be fine with Launch-MG42-With-Supression-Bulletin levels).
For gameplay, this would be good; it would stop blobs of death running right up to an MG. It would also make the MG an area-denial weapon, like it's supposed to be and re-introduce the clever placement of MGs. It also wouldn't be that tricky to beat, as simply flanking it, or spreading your units out (outside the AoE) would nullify it. Queue mine usage and proper support.
For realism, it just makes sense; if you're an MG-gunner, and you see a huge blob of guys in front of you (right in your direct LoS firing arch), you're going to hold down the trigger until the barrel melts and/or you run out of ammo. Against a single guy or two, you'd want to conserve ammo and fire in short bursts.
Alternatively, you could just give a movement and damage nerf (negative zeal, anyone?) to blobs. The guys at the back aren't going to be able to see/shoot anything, and the guys in the middle will have no idea where they're going.