Well,the easiest POD is simply to have Hitler not pursue both objectives simultaneously by splitting Army Group South in half. The extra manpower and machines should be enough to take Stalingrad in the first assault-which came pretty damn close to succeeding in OTL with the vital Soviet ferry landing stage being brought under direct German machine-gun fire for a time.
So, Stalingrad is in German hands by early October. The Germans next move will probably be to eliminate any lingering Soviet bridgeheads over the R.Don. Next, they'll go for seizing the East coast of the Black sea along with Maikop,Grozny and Baku. Road infrastructure was pretty terrible in the area at the time so that will take a considerable length of time. Small Motorised units will probably speed ahead to fight off soviet remnants whilst engineers build up the logistic routes as they go. An earlier fall of Stalingrad probably means no surprise capture of Maikop and it gets its oilfields trashed,ditto for Grozny. Then, we come to Baku which will probably now house the surviving soviet forces in the Caucasus. Baku was supposed to be captured intact and if memory serves had at least some defensive belts constructed by the Soviets to protect it. Assuming the Soviets make a final stand here and can be at least partially resupplied[Via the Caspian?]they stand a good chance of holding for a while, perhaps creating a mini Sevastopol type situation. Baku would take a while to crack since the Germans are at the end of a painfully long logistical supply line and want the oilfields as intact as possible.
So, a seige of Baku could rumble on well into late spring 1943.
On a wider scale-Operation Uranus is butterflied away by the victory at Stalingrad.
Zhukov gets the extra million plus men that would have been used in Op.Uranus and probably uses them in a scaled up version of Operation Mars. Army Group centre will lose ground although nothing of major importance.
Morale/manpower constraints and fuel shortages impose themselves on the Soviet forces by late 43.
Ultimately, the war ends with the Germans still deep within the Soviet Union as the first Atom bombs are dropped on the Fatherland.