1. C96 "Red 9"
Due to the ingame C96 using 9mm Parabellum ammunition I would like to suggest that you change the ingame model to reflect that real 9mm C96s had a number 9 branded on the hand grip and painted red to warn the user to load 9mm instead of 7.63x25mm Mauser ammo. Or at least add it as an additional skin.
2. Shotgun for the US (M12)
A very popular shotgun used during the war with about 2 million produced. Some Pictures:
Also here's my idea on how to implement it fairly:
3. Shotgun for Germany (M30 Drilling)
It was a triple-barrelled break action shotgun with the third barrel being used for a Rifle round (Most of the Above Information will apply)
I'll keep adding more if I think of anything else