There is enough historical hardware used by the axis and allies on both the eastern and western fronts to make dozens of complete factions. Like Katitof mentioned each faction represents a "theme" and not necessarily the entire nation.
Ost and Sov are modelled from a post-Kursk 1943, but you could easily make 1941-42 era Soviet and German factions with weapons yet to appear in-game like BT series tanks, T-26s, Panzer IIIs, and towed 88mm Flak cannons. Making such factions competitive with the likes of 1943-45 technology represented in the existing factions would be the main difficulty.
The British, or perhaps more appropriately, the British, Canadians and other commonwealth could face off against the German/Italian Social Republic armies in southern Europe.
I believe I don't need to explain why japan will never happen(vastly inferior everything that wasn't in the air or on the sea).
eh...I don't know about that. It's not like the Marines and US Army troops were exactly well-equipped in battles like Guadalcanal...armoured warfare was relegated to secondary status in the Pacific anyway, with the US deploying most of their armour in Africa/Europe and the Japanese in China. Stuarts match up well enough against Ha-Go and Chi-Ha tanks...and for the occasional Sherman the Japanese can use this to deal with it.