WI you could assemble the best possible submachinegun using 1943 technology?
Borrow ammunition from one nation, magazine from a second, stock from a third, etc.
To keep this within the definition of "submachinegun" please limit your choice of ammunition to a short list of pistol cartridges that were widely available in 1943: 9 X 19 Luger, .45 ACP, 7.62 Tokarev, etc.
Also keep in mind that SMGs traditionally serve two different roles: infantry and PDW. Infantrymen need SMGs that are good for closing with the enemy out to a maximum range of 100 metres.
Meanwhile, Personal Defence Weapons were issued to specialists who's primary duties did not require closing with the enemy: artillery men, engineers, drivers, cooks, etc.
My fantasy SMG vaguely resembles a STEN Mark 3 with a few Mark 5 parts, but the configuration is "bullpup." The barrel is a full 18.5 inches (50-ish cm) long, but the total length is barely 24 inches (53-ish cm) long.
The pistol grip is near the muzzle, with a trigger-guard that looks like it was stolen from a TAVOR and is big enough to accept winter mittens.
The selector lever/safety is within a thumb's length of the trigger.
It has just enough wooden/Bakelite furniture to prevent soldiers' fingers from freezing to the gun.
The sheet metal receiver conceals a 15 round tubular magazine (PDW) but it will also accept 30 round stick magazines (9X19, double-stack and double-feed) for door-kicking.
Empty cartridges eject downwards.
Bayonet lugs will accept whatever bayonet is issued to accompanying riflemen.