No faction ever should have stock heavy tank, its just broken.
No faction ever should have unlocked weapon upgrade or grenades without paying additional menpower and fuel.
No faction ever should have access to map hack vehicle that ignores LOS blockers.
No faction ever should have access to durable forward structures you can reinforce at.
No faction ever should just have access to anti tank gun from start of the game, regardless of tech.
No faction ever should be allowed to upgrade weapons on enemy territory.
Or, now pay attention, all factions should have something unique for them that no other faction gets.
+1000 to all but to this one:
« now pay attention, all factions should have something unique for them that no other faction gets.»
I would replace it by:
All factions should have access to
all the basic tools in all field, but each faction should have
better upgraded tools in one field that the others factions
don't have access to.
Rough examples

special expertise field)
OKW : Super armor= K tiger
Sov : At-gun = At-gun with indirect capability
Wer : Support : Mg42, mortars the best.
Brit: Scouting/commandos : brent carrier flamer, commandos (no sure what best in brits) ? help me ?
USA : Leadership (lt,capt, maj) (no sure what best in US) ? help me?