I prefer how it was done in ME2, it was like regardless of class Shepard had heavy weapons training, I liked it more than the conveniently located weapon in ME3 anyway.
I'm not sure if I want our Pathfinder to be a heavy weapons expert too though, I think that I'd rather just not have them in sp and instead it's a class in mp.
Boss weapons were an acknowledgement that some weapon and power design decision weren't fully tested in ME3.
Heavy Weapons should be just that, Heavy; and the Soldier class should have the best carry accordingly.
2's system was pretty good; no heavy weapon was all things to all enemies, but there was a weapon for almost every single situation, with only the CPC as a catch all with the caveat you were exposed from cover and could target a single enemy, whilst the blackstorm was essentially a portable warp+singu combo exploder, but did crap against vehicles and armor elites, etc etc etc.
If heavy weapons return (and they should, why the hell would you go to a new galaxy without heavy ass contingency plans), they should be a weapon, not a get out of poor gameplay choices free card as they were in 3.