I can't see this happening unless the USSR felt it was either in a) a position to lead the postwar order (no US/Western superpower), or b) that it had so little chance of being able to spread communism to other countries that it decided to simply grab what it could and build socialism in isolation (while waiting to rise another day). IOTL the USSR established nominal federalization instead of Chinese-style centralized administration in part because in the 1920s it seemed to many that the capitalist world was on its last legs and that they might soon have to welcome Polish, German, Romanian, Turkish, etc. SSRs into the family.
Post-World War II, a is not going to be the case with the US being what it is, so b is the likeliest option, assuming the non-Soviet powers are vehemently anti-communist. I don't know what could cause this; perhaps a concerted strategy on the part of the western Allies to undermine Soviet influence in Europe and Asia starting in 1944 (after success of Overlord) could be such a blow to the USSR's postwar position that it decides to knuckle down on whatever territory it has claiming the need for collective self-defense or something. Surely the CPSU is capable of the ideological airbrushing needed to portray this as something other than an imperialist project.
Such a USSR would be much more repressive and likely to maintain its totalitarian system for a longer period of time. I can see it maintaining a relatively low profile internationally while moving to take advantage of splits between other world powers.
Internally, the enlarged USSR would have several more SSRs, including a Chinese (Manchuria and possibly Xinjiang) SSR. This USSR would have permanently bad relations with the Republic of China for not only keeping Manchuria communist, but also having it annexed into the USSR. In Eastern Europe, I can see the Soviets holding less postwar territory as a result of Western conspiracy late in the Second World War, perhaps being confined to Poland, Finland, Bulgaria, Hungary, and Romania.
The Soviets would have a big headache assimilating these large populations not traditionally controlled by the Russian Empire, but it could be done with a good amount of violence couched in the rhetoric of anti-landlordism and anti-nationalist chauvinism, similar to the "de-extremification" campaigns used in OTL mainland China to justify persecution of the Uyghurs. I can see local resistance in Manchuria — 40 million people, basically a Ukraine of the East — being worn down through man-made famine and campaigns of "de-feudalization" carried out by the existing communist Chinese cadres, and accompanied by a Soviet-led period of reconstruction and urbanization to mold a politically obedient middle class. It is possible, if not likely, that all languages spoken in the USSR including Mandarin will be reformed to use Cyrillic.