Geography is an important question. I'd addressed it before, but Germany needs a land route to attack the Soviets. Figure that mowing over Poland would not be acceptable to the West and would likely provoke a war.
So you have five routes between the Soviets and the Germans:
Finland and Southwest: While Finland has claims on Soviet Territory, I don't see them launching a war against the Soviet Union for them. If the Soviets decide to abuse Finland first, it would probably support this offensive--but that is a contingency that is not likely to happen. The Soviets have little ground to give before Leningrad, but grabbing Ukraine or Moscow with this position is unlikely, and Leningrad is a tough nut to crack.
Baltic States: These nations are not going to support Germany diplomatically, not after the whole Freikorps thing tried to make them German Puppets. Lithuania was bullied out of the Memel Region--but even without Memel, the Baltics would not support a German attack. Germany MIGHT be able attack the Baltics without a reprisal from the UK and France, but it still has the next problem to consider:
Poland: More than any other choice on this list, Poland doesn't want to get involved and it has nothing to gain from a war in the East (Its satisfied already, after the Polish Soviet war of the 1920s). Poland MIGHT give transit rights across the Corridor under very limited circumstances, but the Germans would not have the wide front they enjoyed through negotiations with the Poles. Attacking Poland drags the UK and France into the fight.
Romania: The Pro-Allied nation of Romania is not going to support Germany as long as France is still standing. Attacking Romania is likely to augur French and British Wrath as well.
Turkey: Plausible in a diplomatic sense: The Turks want to have Armenia back, minus the Armenians. Not Happening any way shape or form militarily, as Germany has no way to supply Turkey satisfactorily, nor could a German army in Turkey hope to drive all the way from Kars to Moscow.
Geographically, WW2 would be a farce if the allies didn't get involved early on, and there's no way it could happen without them getting involved.