Soviet American Solidarity
I've banged this drum before- say Stalin dies during the defense of Moscow.
Molotov, Beria, or some other moderate has no objections to WAllied troops on Soviet soil. Instead of spending three years waiting for WAllied troops to do more than take babysteps pinging the Atlantic Wall and wrapping up N Africa- they see Americans, Canadians, Brits, Free French, Polish etc troops dying to help liberate their homeland.
Americans get a clue about decent tank tactics and deep-battle strategy, maybe even adopt the T-34 and ppsH-44 but mostly, see the biblical devastation wrought by the Nazis on Russia, Poland, etc. firsthand and respect for their Soviet comrades in the drive to expunge the Nazi threat not just from Russia but the Earth shoulder to shoulder.
I agree it's ASB on several levels. Stalin'd never allow it. The WAllies'd be appalled by the carnage on the Eastern front and terrified of being commanded by the Soviet front commanders with their casual acceptance of casualties to accomplish objectives. It'd be logistical nightmare to accumulate enough troops and gear either through Archangel, Vladivostok,
and/or Iran to make an impact.
However, imagine the impact of hundreds of thousands of Allied troops on Soviet society. Instead of feeling like the world's ganging up on them,they see the world coming directly to their aid in their darkest hour.
Since the West is coming from the East, there's no sweat as to who gets to Berlin first. If anything, US/UK bombers based in Russia are considerably more effective at taking out Ploesti and other key objectives. They'd be running headfirst into major LW resistance, but they did anyway.
Being able to sandwich them between UK and USSR-based bombers, Germany gets strategically strangled quicker.
VE Day could happen in 1944 if the cards are played right. Soviet formations could end up replaying what tsarist troops did in defeating Napoleon, exposing hundreds of thousands of Soviet troops to Western Europe. Since there's lots of Western troops around, the Soviets don't feel quite as free to exact revenge on German civilians.
YMMV as to whether the Western troops object, turn a blind eye, or join in. Knowing human nature when confronted with such horrors, probably a little of all three.
The USSR is no longer an isolated pariah nation. Allied engineer units work wonders improving the Soviet rail, road, and air networks. Many Western volunteers and opportunists come to the USSR to help rebuild, offered incentives to help bring land back under cultivation and rebuild industry wrecked by the Nazi invasion more or less under the Yugoslavian co-op model.
Soviets study abroad and bring back ideas about business management and political freedom as well.
You butterfly away the mutual suspicions that fueled the Cold War. The Russians don't need the Iron Curtain and Warsaw Pact for a strategic buffer. Europe is helped to economically recover from the Urals to Brest from the nightmare of WW2. The US doesn't feel that the Soviets are as much of an alien threat and neither does the USSR view the USA as such.
China is liberated by Allied Soviet-American-British forces pushing through Manchuria some going south, and some continuing on liberating Korea. I'm betting a coup by Left-GMD forces topples CKS and incorporates some CCP figures into a coalition government- Mao dies a mysterious death, Chou-en-Lai survives as do other moderates.
They wind up agreeing on an authoritarian national recovery scheme to vastly improve agriculture, light industry, and educate their population that is far more social-democratic in nature than OTL Great Leap Forward.
There will be commerical, technological, and political rivalries, but not the mutually suspicious armed camps threatening nuclear holocaust to billions.