Well, since we're talking alternate Fall Blau:
**Do note, if anyone would have proposed this to Hitler, he probably would have been shot on the spot**
How about an offensive aimed at establishing a strong defensive barrier on the Don riverline?
From Rostov, all the way north to Voronezh, then north-west through Livny, to anchor on the existing German lines at Orel.
There is great potential here for a defense in depth. Alternate defenses could be dug at the Donetz, as fall-back positions.
From a line along the Don River, the Germans would be threatening Stalingrad, and thanks to their earlier conquests, they're still threatening Moscow and Leningrad as well (then again, maybe Leningrad could be taken in 1942).
Let the Soviet winter offensive come, and then go for a final offensive in 1943, where-ever you like.