you know what you should do
study the film "nothing new on the western front". it was a controversial film adaptation of a book by the same name by a former ww1 german soldier, that provides a realistic, bleak and authentic look into ww1 but just war in general.
also i have an idea for a mission, assuming youre doing it cod style. try to get a civilian mission, i find those to be extremely interesting.
an idea for a fictitious mission would be:
the screen goes black. you are introduced as german private jacob gable, a civilian who has been recently drafted. the screen shows you walking out of your house down to the recruiters who wander during an honorable parade in celebration of veteran soldiers. fast forward a week. as you are given your uniform and rifle at the training camp, you partake in a drive to the western front where you are one of thousands of men. you are fed and are put to rest. you wake up to the sounds of gunfire, yelling and explosives and are picked up by private otto meyer. you quickly arm yourself and text comes on the screen. it reads. "welcome to war. you must fight for your survival and your country. you are not expected to survive." you quickly run out of ammo, and are forced to take cover or accept your inevitable fate as another casualty in a big war.
i stole this bleak hopeless idea from a mission in cod: world at war, which is heralded as a very genuine, authentic and dark shooter game that is not afraid to appear dark, muddy, cynical or legitimately depressing.
i honor your idea to make a ww1 fps game as it is not an idea often executed. if you would tell me the title when it is done i shall purchase it quickly and work through the story. i wish you good luck and fortune.