Its called hypothetical scenarios! Its very easy to imagine the US or UK fighting against the Soviets... granted it would be rare to see US against UK, but many of the alliances changed throught the war, and many others shortly thereafter.
Then you just have tournament/quick game type play where realism matters none.
Anyway platoon lists that I'd want to see in WWII:Evo (to start with, I'd like to see almost everything eventually).
US: Rifle, Airborne, Armored, Tank.
UK: Rifle, Airborne, Tank.
German: Volksgrenadier, Panzergrenadier (Armored), Panzergrenadier (Motorized), Fallshimjager, and Panzer.
Soviet: Strelkovy (Conscript), Strelkovy (Gaurd), Tank (Conscript), Tank (Gaurd).
Of course some units would work in more than one platoon, IE US rifle models would be the same for Rifle and Armored Platoons h/e they should have different weapon options, and the armored platoon rides in halftracks. Also, some divisional assets (like AT guns, off-board artillary, and a tank [if playing an infantry platoon] or infantry [if playing a tank platoon]) would be available for each army (though maybe different for each unit, Airborne/Fallshimjager wouldn't have access to the same things as a Tank company)