Except there is nothing in the rules books that says "No Heavy for USF".
You mean except of what devs are saying since WFA alpha, which means nothing except intended design is on the way?
This is a common thought trap people get them selves into, just because a faction doesn't have something doesn't mean that faction can't be allowed to have it.
Nothing against soviet stock upgrades/panther like tank, USF heavy tanks, OKW arty or normal resource rates, Ost light tanks and stock upgrades being made into doctrinal or side upgrades.
That still doesn't mean it will happen.
Just because something isn't strictly forbidden doesn't translate to "OMG X CONFIRMED".
OKW have no on map arty, because they are designed not to have it.
Same with USF heavies.
Same with Sov upgrades.
Same with Ost light tanks.
Not all armies are supposed to have everything.
Not hard concept.
But as you've said, armies should be able to do similar stuff differently.
OKW heavy arty is non doctrinal Stuka. You can deny it, doesn't change the fact that its equally strong as LE.f.h or ML-20 or Priest, except much more precise and non doctrinal, so you're striped from that argument as well.