Its hard to make the Soviet armed forces worse positioned or prepared to meet the invasion than they were. You could do something along the lines of Stalin gets overthrown, the regime collapses and/ or sues for peace, and ironically the best POD for that is probably no purges.
The Barbarossa plan could be improved on, though given the size of task I think earlier commentators are being over-critical of the plan itself. The Germans would have to get everything right. But something on the lines of a two year plan, where the goal in 1941 is to seize Moscow (disrupt Soviet politics, communications, and logistics), and in 1942 the goal is to take the resource rich areas to the South. They sort of do this anyway, but in this version there would be no attempt to take Leningrad, with Army Group North consisting of two infantry armies operating as flank guards from the start, and probably no diversion of Guderian to take Kiev in 1941, though if the panzerarmee and air assets are removed from Army Group North at the start, adding them either to Center or South probably makes the diversion unnecessary or less of a big deal. Taking Moscow first and then Ukraine would have worked if they had actually taken Moscow in 1941. There is even more room for improvement in the planning for Case Blue, again same idea, concentrate on securing the lower Volga first and cutting off the oil fields, then go for the oil fields later.