It seems like the next stand alone ,after two new DLCs, in Achtung Panzer series is going to be Khalkhin Goal where the Russian and Japanese armies fought . It is not a well known battle but has important impacts on overall outcome of the war as Japan had to cancel its plan on Russia and focus on the Pacific theater.
I am not sure if it is the best option but it is going to be an interesting one.
It is certainly also studied for its contrast of Japanese vs. Russian strategic doctrines and high-level command vs. low-level initiative/adaptation. It is also interesting as one of the early tests of armoured mobile tactics (very important 3-4 years later)!
In English it's named Nomonghan not Khalkhin Goal
Nomonghan is well known balttle, many books about it.
Funny, I always though it was Kalkhin Gol....