Realistically:
One of germany's hare-brained schemes for advanced air defense actually works, and the Alies don't deliberately or accidentally stamp on it before it goes live. Also Hitler doesn't screw it up.
What scheme do you have in mind exactly ?
Also, as to my take:
Soviets overextend themselves trying to both stop Mansteins relief force and get to Rostov at the same time. They achieve neither and most of the 6th army escapes. Von Paulus either stays behind or is court-martialled and shot.
More effort put into Stalingrad means the Germans have written of Tunisia. Thus no quarter million men trapped and no free training for US forces, plus Fredendall is still in charge.
With more troops in Sicily and the americans inexperienced and led by an idiot, op. Husky ends in disaster. The allies spend the rest of 1943 conquering Sardinia and Corsica and may or may not succeed in occupying some worthless Greek islands.
On the eastern front, Mansteins backhand blow works a couple of times untill the soviets wisen up to it. From then on, the germans adopt an even more defensive mindset and don't even launch Citadell. They do however execute a succesfull
Operation Eisenhammer (beeing closer to Moscow and all). Work on the Panther-Wotan Line starts earlier, also helping a bit. Month by month, soviet manpower shortages become ever more severe, while Hitler somehow wisens up and stops using "no retreat" orders when the situation is clearly unwinnable, thus sparing many troops from capture.
In 1944, Overlord is narrowly defeated thanks to a number of small butterflies, but the allies do land in southern France and slowly make their way north.
In the east meanwhile, the Soviets take the Ploiesti oilfields in late Decembre 1944, though the axis do manage to block the Carpathian passes. They also reach the 1938 polish border.
By late spring 1945, the allies have once again landed in northern France, while a soviet spearhead has reached the Vistula south of Warsaw. The Red army is also advancing south into the balkans, although logistics hamper the operation. The Soviets are however suffering from acute manpower shortages.
By late autumn 1945, the nazis have been driven out of France and behind the Siegfried line. They do succed in extracting a large percentage of their forces though and also devastate an allied attempt to capture bridges in holland and Belgium via paratroopers, while also cutting off and destroying an entire armoured division thanks to selective flooding of the area. This is followed by their last major succesful counteroffensive, which thouroughly defeats an overstreched soviet attack towards the Oder, thanks in part to most soviet airfields being unasable due to the autumn rains, while the Luftwaffe operates from the concrete airstrips around Berlin. This is also the last time the luftwaffe enjoys aerial superiority.
Winter 1945 sees a grueling attritional battle along the Vistula in the east and the Siegfried line in the west. Come spring 1946, Germany's enemies have reached the Rhine and Oder rivers.
In late spring 1946, with the nazi state collapsing, the allies encircle the Ruhr area while the Soviets take Berlin. Hitler commits suicide and the european axis surrenders.