Ok, I’ve just found the perfect scenario for a Left 4 Dead campaign:

Zombies? Check! Nazis? Check! WWII era weaponry? Check! Of course, this scenario would require a huge effort on the part of the developers: New player models (with winter coats, boots etc), new zombie models (Nazi uniforms) and new weapon models (I figure the survivors would use WWII era German weaponry that they find. Bit of a stretch to expect it to be in good working order but hey, there are zombie Nazis stumbling around! Anything is possible!). Plus all the level work needed. But I bet the result would be well worth it!