We all know that during the war, various items (including weapons) were kept as 'souvenirs', taken off dead enemies and kept by surviving soldiers. The most famous example of this, one most of us are probably familiar with, is American soldiers recovering Lugers from felled German officers. My suggestion is this, and I think it adds a nice touch of realism - we do a bit of research to find weapons that were commonly stolen from dead enemies, and occasionally, through either random spawning (think the M1 Carbine/Shotgun scenario) or through random pick-up kits placed by the devs in obscure parts of maps, a sort of small reward for the more adventurous soldier.
Say, for example, you might in one of the Normandy maps be off dicking around in the far corners of the forest and come across an American rifleman's kit, only it's got a Luger in it, in addition to the standard knife/Garand/nades/rifle nades. Or maybe in a Russian map you'd come across a kit that was completely German except for a liberated SVT40.
I don't know, what do you think?