I think they would release matched pairs....
p3 J/L vs KV1s maybe?
a KV1e or later b/c cast turret might be a bit too armoured for the panzer.... kinda like the desision in RO i guess.
the p4 long was an obvious choice really to balance a 76mm russ tank, but before that you have a balance problem with the russ use of dual purpose guns vs the ger specialised ones.
So if you need a p3N/p4F1 with an p3 J (L60) to cover both inf support and AT, against 2x kv1 or t34 the germans may have a hard time.
I've always thought that implementing evenly matched yet realistic tanks for stalingrad - particularly in small numbers for CA maps - will inherently be a tough job.
If you went the spg route then matching the short stug is tricky - i don't beieve the su122 saw much of stalingrad unfortunately, leaving the su 76 which would have a big advantage meeting an L24 stug.
However, put in a stug F or later with the same L42 as the p4g and it's the russian that will get hurt badly.
morg is correct BT-7 vs PIII short 50 may work ok, and as i lover of early tanks i'd welcome them - but i think many ppl will yearn for the heavier-hitting stuff as was the trend in ROost