Yours truly BW749 is kicking around several alternate case blue scenario's at the moment (as a couple of my open works will close this week, I just returned from Denmark and Poland tonight)
1. Hitler doesn't divert the 4th panzer army south and they capture Stalingrad off the march in July 1942, turning over security to 6th army and then carrying their attacks farther south. The 4th panzer army accomplished nothing in their move to Rostov, and in Kleist's words "I didn't need the 4th panzer army, and they merely blocked the roads I was using and slowed the progress of my units allowing the Russians to consolidate
2. Hans Hube of the 14th Panzer Division is the first to reach the Volga at Rynok, where instead of waiting several critical days for the rest of the 6th army to catch up (otl) he instead launches an immediate river crossing and establishes his panzers on the east bank, where they roll up the city... up to this point the primary Russian plan was to abandon the city, let the Germans overextend, and then attack them in the winter.... his delay allowed the Russians to pour in forces, reorganize themselves and realize the Germans where set to storm the city in a blood battle.
3. Manstein's 11th army is sent to be in strategic reserve following behind Army Group A as he wanted, allowing the 17th army to branch out and allowing the 1st panzer army's flanks to be more secure.... very likely they at least reach the Turkish border at Batumi... however the butterflies are immense, since the 11th army was in otl sent to Leningrad, where they where able to hold off the Russians in the Second Battle of Lake Ladoga (and inflict quite a blood bath too)... without their presence its possible Army Group North is decisively defeated
4. Paulus has a stroke right when the encirclement happens. He is replaced with Seydlitz the senior corps commander, and a Hitler favorite (at the time). Seydlitz orders and immediate breakout to the southwest and is able to restore the front with his own resources
5. Operation Torch is delayed by a period of several weeks so that the encirclement at Stalingrad happens first. Divisions that ended up serving in Tunisia are instead sent to Manstein's Army Group Don which instead of being two weak corps is a full strength panzer army and he relives stalingrad, rescuing the 6th army whilst dealing massive blows to the encircling Russian forces... Africa falls apart by january 1943 with the DAK lost... but the other divisions are still available for service, even if weekened
6. Hitler doesn't fire Bock for not following orders at Voronzeth, and Army Group south doesn't get broken up into two seperate army group's being commanded by OKH/Hitler directly. With the failure to capture the city, and the encirlment Manstein is brought in to command army group south and is able to rescue the 6th army with otl resources by the skin of his teeth
There are a number of other variants ive dweled on... but these are the ones where I think there is some meat... at least one will turn into a tl this year