I don't know if this would be a good fix, but can't they just make it so vehicles/weapon teams don't give negative zeal? And so that negative zeal would only trigger once a third squad gets close (something something two's company, three's a blob)?
Like for example: a lone rifle squad. another rifle squad comes close, no debuffs given. a third rifle squad comse close, a debuff goes out to all squads - "+20% received accuracy, -20% accuracy". a fourth rifle squad comes, the debuff is increased "+35% received accuracy, -35% accuracy" for all squads. the debuff stays until there are no more than two squads close together.
Or is this just not a possibility for CoH2 engine?
They could definitely make it so vehicles and support weapons don't provide an aura - that should be as simple as simply not assigning the aura to the specific unit. Receiving the aura would be a bit trickier - preventing vehicles from receiving it would be easy, as many auras already do that (arty officer, landing officer ability, etc.). Preventing crews from reviving the aura might be a bit trickier, since (iirc) they're still infantry. It might be possible since the USF "on me" is essentially an aura that only effects non-crew infantry, but I really don't know the specifics on that.
Getting the debuff to apply only when there's MORE than 2 squads would be quite tricky, since there's nothing in the game that works in that manner. What could be done is have each squad provide a non-stacking positive (and non-self) aura as well as the stacking negative aura. Essentially, when only 2 squads are around, the positive aura would cancel out the negative aura, resulting in no change. However, with 3 squads, the positive aura would only cancel out one negative aura, leaving the balance at "-1".
This is of course an incredibly messy way of doing things (I'm sure every programmer would be screaming), but it would work (I think...).
I think that might be possible. A fusion of that and the in-cover exemption Doomlord proposed would work fairly well. The exact nature of the debuff is up for debate of course (reload time for AT blobs?).
The exact nature of the debuff would be quite tricky. An increase in received accuracy would make blobs less resilient against other infantry, but it would result in mortars (or any other AOE) being insanely effective. This might seem like the desired effect (blob = mortars become incredible), but it would mean that map choke-points would become incredibly risky: the central VP on "Trois Ponts" would become almost impossible to re-take, since there's no green cover, and a single squad would almost never last long enough to capture it.
A reload speed debuff could work, but it doesn't really prevent blobs from being incredibly powerful. Even with a -50% reload speed (way too high), a group of 4-5 shrek volks (last patch) would still insta-wipe a Sherman - they'd just need more time to do it again. For anti-infantry, it could work though.
Personally, I think the best choice would be a combination of an accuracy nerf and possibly a received suppression increase. This would mean hand-held AT would be less powerful in blobs (large volleys would likely miss a lot) and infantry weapons would have a much lower DPS. In addition, suppression usage (i.e. the counter to blobs) would be more effective, resulting in an increased incentive to properly micro and flank. However, with this debuff, surviveability wouldn't really be changed - unfortunate choke-point ambushes wouldn't wipe large (unintentional) blobs, and mortars wouldn't make central VPs uncappable.
If I knew how to make such mechanics changes, I'd make a test-mod (might look into it anyway).