As for China, I think that diesels are good, but they'll alslo need strong turbo 4 cylinders (most likely something based on the new 1.6T they've debuted already) and it will need Buick-like interior noise levels.
If they were to go so far as offer such a product in the US market they'll need (in my opinion) to offer the product with AWD, have it priced accordingly so as to not create too much overlap with the ATS, make it fun to drive, and make it convincing for luxury buyers. Other GM products like the Verano are solid and convincing luxury/near-luxury vehicles. They'll need to offer something head and shoulders above it to make it stand out in comparison to corporate-cousins like the Verano. Furthermore, while some folks have detracted the Verano for some interior materials or powertrain issues, a future sub-ATS vehicle can't have any kind of similar faults. In fact, one good way to think about it is, any future sub-ATS would have to pick cup where the Verano GS leaves off.