Almost all these countries have the same problem: BFV features full factions, with full cosmetic outfits for each. That's all good for Britain and Germany, who fought throughout the war and will feature in almost every map... But Poland was defeated in a month and France was lost immediately after the Phoney War. Sure they fought after that, but alongside other major factions, eg the Polish who fought in Tobruk with the Brits. Finland just... Didn't really fight outside its borders, and I don't think the community would be too happy with multiple Finland maps - it'd be like several Narviks and Fjells. So it would be an odd decision to give each of those countries a faction.

That leaves Russia, Japan, and China. Russia was very active in the early war... Against Finland and Poland. Without their factions, who would Russia fight? I expect we'll see them in the later war, against the Germans.

So China and Japan: the Sino-Japanese war was effectively in a stalemate until the end of 1941 when Japan attacked everything it could see and brought the US into the fight. I expect we'll see the Japanese when we see the US, and given the timing, I expect that'll be in the upcoming roadmap. Perhaps China may appear later, but at this point in the timeline they and Japan would've appeared in maybe one map.