I suspect a Soviet attack on Norway/Sweden/Findland set during the attack on Germany would be natural.
Now that would be interesting!
"I sincerely believe the banking institutions having the issuing power of money are more dangerous to liberty than standing armies." - Thomas Jefferson, letter to James Monroe, January 1, 1815.
Sino-Soviet would be incredible. New terrain, non-European maps -- oh boy a whole new theater. Think about East is Red (SPI) and its sequal. Those games were a blast, as well as the Sino-Soviet wars in Operational Art of War. Game could cover a wide timeframe too, from late 60s to later, even contemprary.
While I think that the hypothetical (and extremely unlikely) Sino-Soviet would be interesting, I think that the actual Soviet offensives of 1943 are ripe pickings for an excellent PzC title: