Just another brain dump. These would be interesting weapons to see for certain infantry units.

Ribeyrolles 1918
Although it was only a prototype during WWI, there's a chance that this French "machine carbine" (an assault rifle by today's standards) could have seen some action at the start of WWII in desperation against the German offensive. Maybe a good option for an elite unit?

Fedorov Avtomat
Developed in Imperial Russia during WWI, this automatic rifle saw further action in the 1939 Winter War with Finland. Gun experts are still debating whether or not it could be considered an assault rifle due to its 6.5x50mm ammunition sitting right smack in between intermediate and rifle calibers. Certainly an interesting choice for assault units.

Marlin 1894
A revolver-caliber lever-action rifle that was issued to certain Allied home guard units. Pretty self-explanatory.

LAD MG
A Soviet prototype belt-fed SMG. This would be an interesting combination of intense close-quarters firepower and suppressive fire for an elite unit.