I know that the romanian army was absolutely desperate for AT guns able to knock out the T-34 at a reasonable distance. As stated the germans were very short of everything themselves, so the gear must come mostly from somewhere else, question is where? The romanians and hungarians could theoretically commit more aircraft (actually they could easily double their respective commitments i would think, though that would leave little in the respective countries) and whatever AA, AT and field guns of every kind they still had in their countries and more troops, though as probably known, they were both retaining large amounts of troops and gear facing eachother over Transilvania! Something like the TACAM type vehicles would be useful for both, if somehow they would have managed to convert a significant amount by the time of Stalingrad.
But probably the most important factor is actually having the germans heed to the signs of the impending soviet offensive and take appropriate and sound measures (they did received warnings about this impending offensive from romanian recce among others).