If the Soviets were being led by a fully rational human being, then
I think it would be incredibly unlikely for them to lose.
Unfortunately for them, they were being led by Stalin.
The scenario that seems somewhat possible to me would be something like:
1. As an initial divergence, Germany decides to emphasize relations with China rather than Japan.
Many Germans were more sympathetic to the Chinese, and they already had military advisers helping the Nationalists.
2. The catalyst of war stays the same, but for one reason or another, Germany closes the noose, and captures the British and French armies at Dunkirk.
3. Japan, seeing that France utterly defeated, and Britain is on the verge of collapse, attempts to repeat WW1 on a grand-scale. They invade the French English and Dutch East Asian empires, and rapidly make progress, as they had historically.
4. Britain is desperate. Germany has taken hundreds of thousands of their troops captive. Japan is crushing them in the East, and Indian independence leaders know that the British Army is gone. This desperation lingers as the Germans begin looking for a formal surrender, and the Japanese make more progress every day.
5. Japan takes Singapore, and their officers either lose, or release control over their men. Singapore gets the Nanking treatment, and everyone finds out about it.
6. During WW2, Japan and Germany had an odd habit of having people who cared about human rights in each other's territories, but not in their own. Germany, already disliking the Japanese because of their brutality toward China, are even more hostile now. They offer a deal.
7. Generous peace with Britain. The British Army and their supplies returned, in return for a Bilateral peace treaty. Principled British politicians oppose this, they are shouted down by a drowning nation, looking for salvation.
8. The peace, is arranged, and agreed to in secret talks, while Germany and Britain gear up to strike down what their leaders consider to be the worse enemy.
9. It's 1941 mid spring. British ships sail into Channel ports, and take their army home. They continue preparation for revenge in the East, at the same time, Barbarossa begins.
10. The USSR had every reason to believe Germany was planning this IRL, but Stalin refused to believe it. The Nazis were considered an inherently proletariat movement. Additionally, the Soviets had somewhat favorable relations with the Chinese Nationalists. In a world where the Germans have still crushed France and Britain, and are aligned with China, I can't see Stalin changing his mind.
11. The war goes roughly the same as it did IRL. There's fewer distractions in the West to pacify, but the Germans didn't keep the British equipment. It evens out.
12. Japan is facing the full might of the British Empire, and is almost certainly being embargoed by the USA. If Japan attempts or has attempted Pearl Harbor, then they fair far worse than in OTL. It's difficult to predict their actions in this scenario.
13. I think that many people in important positions in FDR's government would remain sympathetic to the USSR. FDR's hostility toward Germany had little to do with his love of the British Empire. I do not think they would be able to justify nearly as much, and without Japan as an ally, Hitler doesn't have the opportunity to drag Germany into a moronic war with the US.
14. Stalin is the key to the Eastern (now only) front. His will keeps his country fighting, and his madness will hold his country back at the same time. Stalin stayed in Moscow. If for some reason Germany sieges Moscow, and something goes wrong, the USSR dies with him.
I think it's up in the air.
...
Anyway, I won't say that this scenario is particularly plausible, but it was kind of fun to think it though.
Since that's what AH is for, I figure I'll share my interpretation of this scenario.
