The more rare weapons the better. As long as it theoretically could have been used I see no problem in including it, Provided, that only a hero class gets to use it and the average player will use it maybe 1 time in 20 hours play time and see it very rarely.
The whole point is that it reflects its rarity in real life, yet it is a nice little addition that you will feel excited about using when you get the rare chance.
The G41 and the Mb whatever should definetely be included and anything of that ilk , I would say the line starts to blur at say the FG42 seeing as 50 were made at that time and were still being tested in the factory. Its not inconceivable that Fg42 was used by some german soldier somewhere in jan 1943. Anything that was designed after feb 1943 is just out no matter what, anything before....well whos to say?
Another interesting addition could be the beretta 1938 submachine gun. Produced and used by Italy, it found its way into the hands of many german soldiers and was quite a nice little gun.
I know many history buffs would almost die if they saw an early model fg42 in game, but, since if implemented correctly you would only see/experience it every 20 hours game time, its not going to give you TOO many coronary infarctions. Its a neat addition to enhance play value, give a little flavour to the game and make being a hero give you access to some truly rare items.