Perhaps not a Supply Yard, per se (it requires new art content) but global upgrades.
I dont like OF's design, but the PE upgrades make sense to me. Get to fast T3, want those T-34's? need a small upgrade to purchase them. Same, perhaps with SU-85. These, along with perhaps a slight buff if necessary can help slow down the game's escalation.
Not sure if this would work with the Axis, since their escalation is more structured by phases, but we do need something to promote extended early games and branching.
EDIT: Plus, putting this into the game only requires the creation of a couple of icons in the UI for the upgrades, as far as assets go.
Exactly. I mentioned this before:
T1 upgrade - unlocks sniper
T2 upgrade - unlocks ATG
T3 upgrade - unlocks T-34
T4 upgrade - unlocks SU-85
The T1 and T2 upgrades being +-40 fuel and T3/4 being perhaps 60 fuel (this would mean that T3 and T4 building costs should go down, so a fast T70 hits the field at around the same time an M8 would).
This way the early game would last a bit longer, and have teching actually be interesting. Wehr tech might have to be slowed down a bit to balance it out though (thinking of a flamerHT being on the field faster than the ATG, since almost every wehr player goes T2).