the challenge for today is I believe an interesting one. Your task is to find a way to wank things into favor for the Soviets during WW2 as much as possible without things going into the ASB territory. The goal at its farthest is for say 3/4's of Europe to be under the Sickle and Hammer by the end of WW2.
That's actually fairly modest. OTL, the USSR controlled about 63% of Europe, more or less, at the end of the war (including Yugoslavia and Albania).
Remember that 40% of Europe is Russia to start with; another 10% is the former Soviet SSRs in Europe.
To expand Soviet control to 75% requires about 1M additional sq km.
Finland, the rest of Germany, and Greece is almost enough. Throw in Austria and a slice of Italy, and there you are.
How to get there...
1) FDR has a heart attack and retires in 1940; the nomination goes to Burton Wheeler, who defeats Willkie. Wheeler, an extreme Isolationist, bars any U.S. aid or support of the Allies.
2) Stalin anticipates the German invasion in 1941, instead of insisting it can't happen. Soviet forces are fully alerted, and the initial Axis victory is merely substantial, not cosmic. The Axis forces stop E of Kiev, W of Smolensk, SW of Leningrad.
3) Soviet forces roll back the Axis starting in late 1942.
4) Meanwhile, assured of no U.S. intervention, Japan seizes SE Asia, crippling the Allies (that is, Britain and the Commonwealth).
5) Soviet troops conquer Finland, and drive to Greece, Austria, and western Germany. Partisans overthrow Mussolini; Soviet troops occupy northern Italy. Britain, which has just barely hung on with no Lend-Lease aid and has to fight Japan alone, liberates the Benelux, France, Norway, and southern Italy as Germany collapses.