In the past I've suggested a few reasonable nerfs for the Heavy/HMG combo, including:
Remove dual stream Rep Tools
Reduce Dispersion
Reduce Turn Speed
Reduce (slightly) Range

Not all of them to be applied concurrently.

But, since you people are insistant on a more aggressive nerf, I propose this:
Remove Headshot Bonus from the HMG

When I started using Heavies, the HMG was "laser focused" and yet I rarely got Headshot Kills.
Immediately after the Dispersion fix I started getting Headshot Kills 10-15 times per match.
I appreciate a good skill Kill as much as the next guy, but the Dispersion allows the Headshot Bonus to be applied up to ~48 times per second.
Admittedly, that is bad. I don't have the maths on it at the moment, but I'm sure one of you lovely people wouldn't mind posting the hard data...

We (those who suffered through the bad HMG days) have been trained to aim it well. I put the center most point of the reticle on a target and they're instantly dead now. I get rewarded for skill Kills when I'm just spraying into 3-10 clustered mercs.
Granted, I spray high to achieve this because it is effective, but if a nerf is to be applied, this should be the starting point.

Shotgun dispersion isn't nearly that of the HMG.
Aiming for the upper chest from >5m away doesn't equate to a (or several) Headshot Bonus(es).

Best counter to HMG is to spec into it

since when should any weapon be only useful against heavies?

I can honestly get similar effect to the HMG by putting a prototype Shotgun on a fatsuit and damage modding it.

2 seconds to kill anyone with an HMG....

2 shots tops with the proto shotty which takes... two seconds.

Coincidence?

The only difference is if I mount the shotgun people will b*tch about fats running light weapons and OHKing everyone rather than with a (Clearly inferior to everything) HMG that we have had in the past and everyone (Except heavies) was happy with because it was utterly f*cking useless. I ran shotgun fatty during those dark times for a reason.