Barbarossa:
The plan itself is good. The time for it short. Stick to the orignal plan, slow down, and next year you are good to go.
Risky option: Do not do Kiev, go for Moscow instead. Risky, very risky.
The plan wasn't good. Its success critically depended on a single assumption - the ability of the Red Army to effectively fight will cease to exist after the initial border battles. Once this did not happen, it was the end. Barbarossa couldn't be any slower or less comprehensive. Stopping somewhere or giving the Soviets more time only enables them to make more efficient evacuation of the industrial facilities. Besides, at all points in time, it felt that just one more bold strike will bring victory.
Leaving Southern Front alone just enables Soviets to strike north. Besides, the Germans would have nothing left to do with the forces at hand after the Smolensk. They couldn't advance forward before the logistical lines were reorganized and decided to do lateral advance to the south, at once employing the superfluous panzer force from the AG Zentrum and eliminating the threat of flank attack by significant Red Army forces into the axis of advance of AGZ.