OP got me inspired, I'll take a shot at it.
Almost every change has one vision in mind, giving better support to stock USF.
How I would redesign usf.
Teching changes
Major tech changed to 70 fuel. Now has a tech button (50 fuel) like the officer tech buildings have.
Teching major unlocks Thompson only Airborne, giving usf a "clutch" unit for late game to replace riflemen.
Stock Unit changes
LT: Passive ability: when in cover gives it slightly more vision (I&R doc ability vision) giving the unit a bit better spotting for the hmg. Vet 1 ability: lockdown area for 10 seconds, costs 10 muni. Giving more defensive power to the LT to hold off a cutoff for example while you wait for reincorcements. Making it a better support unit.
HMG: Squad increased to 5 men. A bit higher surviveability vs late game elite inf.
M20: Vision increased to 222 and Sov scoutcar counterparts.
Increased health at vet 3, making it just survive 2 AT gun shots.
Smoke removed, replaced by Smoke Screen. Making it more of a supporting role for mainlines and giving it a bit less surviveability.
Major: Forward retreat cooldown reduced to 30 seconds. Costs 120mp + 10 fuel each time to set up. Will provide healing + reinforcement in a form of a medic tent (still delayed reinforcement). Everytime you set up, every player hears a sound. So Axis knows when forward retreat is being used, but not where. Giving USF a bit better mobility late game.
Ambulance: Removed medics crew. Now costs 150 mp. 10 Fuel.
Stuart: Cost reduced from 70, to 60 fuel.
Doctrine changes
Airborne Doc
Pathfinders: 1 CP req. Squad changed to 5
men. 50 muni for snipers rifles, same health crit as JLI. Can't equip additional weapons. Retains camo out of cover like mandos/snipers.
New ability: Smoke drop at location.
Ability changes: beacons no longer provide maphack. Beacons make paratroopers drop down faster like Falls.
This will change paths to a supporting role for mainline rifles. Scout, call in smoke and push up.
Paratroopers: Able to equip LMG, unlike stock paratroopers. LMG upgrade provides Infrared scopes. Giving USF an infantry option vs camoed units.
Heavy Cavalry
Pershing: Has a crew now. Removed crit repair from crew.
Rangers: Gets smoke grenades + incendinary device. Nade removed. Passive sprint when vet 3.
Recon support
I&R Pathfinders: Reduced to 3 men.
Still 0 CP to call in.
Can get rifle nade upgrade for 50 muni, fills up all weapon slots.
New vet 1 ability: Set up in friendly territory for increased muni income.
Beacon maphack ability removed, now removes FOW of an area (AoE comparable to a flare) to better defend against flanks. Only one beacon can be set up.
I&R Arty removed. Now added to the Major. When selection Recon Support Doctrine.
Greyhound: Cost increased to 70 fuel.
No CP, avaiable in LT building after teching.
New ability: Smoke Screen, giving it a better supporting role for rifles and justifying it's fuel increase.
Airdropped AT Combat Group: Airborne squad size reduced to 5 men. Now has snare. Can place mines. When vet 3 passive sniper/mando camo. 90 muni upgrade for double zooks.
Now drops an AT gun instead of a pak howie.
Mechanized
M4A3 sherman: Now gets heatrounds, but with no more pen increase. This will give it great potential to use it togheter with the WC51 mark target ability, which increases pen.
WC51: CP increased to 1, is now a call in.
Now has a crew again.
Gets increased atack range when vet 3, like the M20.
Now gets more vision when vet.
Cost changed to 280 and 10 fuel, comes with the mg.
Infantry Company
M1919A6 Light Machine Gun: Removed from racks, can only be equiped by Paratroopers. LMG upgrade provides Infrared scopes.
M21 Mortar HT: Removed Phos rounds, replaced by flare ability.
Priest: Vet 1 ability replaced by Airburst shells.
Mp cost reduced to 400.
When giving the mortar HT flares, I think it gave it a bit too much synergie with the old vet 1 Priest ability.
Airburst shells blind tanks, giving support to USF armor to better tackle long range heavy tanks.
Armor Company
Assault engineers: now have smoke grenades.
Urban Assault Company
Rangers: Gets smoke grenades + incendinary device. Nade removed. Passive sprint when vet 3.
This will give the USF more clear supporting roles for their stock army.
USF overal strategy: A highly mobile faction which uses combined arms for atacking gaps in enemy lines at the cost of MP bleed and muni costs.