Good there already is such mechanics in game. It's called AoE suppression. All we need is adding AoE suppression to all types of artilery (from mortar to ML-20) and some huge boost to AoE suppression range on ALL MGs. At least 1.5 multiplier of todays values if not 2.0. Some really mild AoE suppression on tank guns can also be considered. These are all quite big high explosive shells, you know.
It in fact has all desired features in it:
1. It works wonders against clumbed squads.
2. It works worse/doesn't work when enemy is in cover.
3. You need to counter the blob, it does not couter itself (like in case of aura).
4. It stops war of blobs (aura doesn't, it just makes blobs disassemble during fights).
Btw it's important to know that one or two mg's should not stop 8 unit blob directed by half minded player. You should need to have at least half of blob's unit count of mg's to counter it.
(2:1 is great ratio in fact and usually it's more as mg's are dirt cheap)
I agree for lots of it except the 2:1 ratio of mgs. Mgs are the hard counter to massed infantry. On close quarters maps having 3-4 mgs is just not realistic because a.) theres smoke and b.) Mgs have to set up before the blob arrives and be facing the right direction. Id counter them differently with indirect fire.