To reduce problems with the player distribution Soviet vs Japanese matches are necessary. This way every nation fights 2 different nations allowing for a more varied game experience and an overall better distribution of the available playerbase in the case that 1 nation is currently under/overplayed.
The Historical basis of course exist in the form of the Battles of Khalkhin Gol (1939) and the Soviet invasion of Manchuria (1945). With the newly added Japanese high tier equipment balance at the high end also should not be too much of a problem.
Since both took place in Manchuria i´d strongly suggest making a map/maps based on the Battles of Khalkhin Gol first since it would be suited for both an early and late war scenario. Maps based on a late war scenario could be added later on.
Lets take a short look at a possible map of Khalkhin Gol:
Since the overall terrain of this area is quite boring (its litteraly just a steppe) I´d focus the map design around the few interesting locations: the river, defensive positions, the village of Khalkhgol, the hills to the north east of the village and possibly camping sites with a bunch of temporary tents either set up by local nomads or japanese/soviet camp sites, a little bit of creative libery sould be fine to keep the map interesting.
for an overview of the terrain in this area here is a modern day image:
(This Image is pointed towards the village of Khalkhgol)
If compare it with historical images i think it would be well suited as map, even if relativly open:
