Ok, how about this scenario

Hitler was sure after Winter War the Red Army was 'kick in the door' Do not forget that Nazis made a pact that gave The USSR the Eastern front. Incl Poland, Baltic countries and Finland I am only glad the Soviets were too sure they would conquer in a heartbeat. If you. say war that is ****. Hitler bought Poland By selllng the other countries instead with Molotov-Riibbentrop deal.

 

I imagine that any country that allowed the Germans a bloodless pathway to an invasion of the USSR would be a later target of the Soviets, if the latter could survive the German onslaught. Stalin had quite the long memory. That might act to dampen their willingness to act to Germany's benefit. That those minor signatories in this hypothetical did not do so in OTL was likely due to Germany's overweening success in 1940; the Balkan countries were overwaed at that point, I think.

Additionally, had the Germans chosen this southern route, they still had to contend with the possibility of the Russians counterpunching into East Prussia and then onward. The northern attack route would place Wehrmacht forces in a position to guard this shortest route into Germany from the east.

I believe Poland was doomed once Hitler and Stalin had assumed total control of their respective countries. One or the other would have tried to cook up another partition, or a unilateral invasion.

 

True, without the German string of victories in 1939-40 Hungary and Romania would have been much less enthusiastic about joining a crusade against the USSR.

A counter-punch into East Prussia? It would have required passing through Poland or at least through Latvia and Lithuania. In OTL the USSR annexed the Baltic countries in 1940 thanks to the Molotov-Ribbentrop Pact. Without the latter I doubt they would have carried on the annexations. Thus a move against East Prussia would have required the outright invasion of neutral countries. I think the Soviets, once facing a German invasion, would have preferred to avoid such a move in order not to forfeit the support of Britain and France. But, as I said, I think a German invasion across the Rumanian border would have been unfeasible to start with.

 

Fair points regarding a counterpunch, though I'm unsure how much support the USSR could have hoped for anyway; I suspect the UK and France may have sat aside to let the two dictators chew each other up, the March guarantee aside, as it was clearly aimed at Germany after Memelland was absorbed. They pretty much stood aside anyway in the OTL while Germany and the USSR partitioned it, and didn't declare war on the USSR as well, only Germany.

We're definitely agreed that the OP hasn't given due consideration to the Carpathians as an obstacle to a southern assault.