I've seen lots of people asking for stat debuffs for blobs over the years. Although I think it would be too heavy handed once you factor in map design and faction differences to simply say 'X amount in units in Y area will induce a 50% aim nerf' or whatever.
However, one thing I think would be an excellent idea would be for larger groups of units to generate more noise within the FoW.
In the same way you can hear a Goliath trundling through your backlines I think the more units clumped together should have a higher ambient noise projected through the Fow.
it could simply be marching boots or special dialogue.
