Yeah I do agree, but sadly even WW2 is getting ignored in favor of this PMC "future" warfare type shit. Honestly it was cool at first, but now it just seems so generic and dated. The guns are all the same fully-automatic bullet spitter, hitscan weapons, that you literally never have to worry about ammo. Guns in WW2 and WW1 were shitty, unique, and all of them had a different feel.
I'm hoping that those World at War 2 rumors are true, because if it is WW1 or WW2 I'll will gladly buy it despite the CoD name.
Now onto what you were saying about WW1, the problem I'm mainly seeing with it is this: the whole entire thing would work like a roguelike, which is a very niche genre. You'll be sitting in a trench hoping you don't lose a limb, get sick, go into shellshock, or getting gassed out. While the idea is interesting, the game would get boring quickly as an FPS, since most of the frontlines never really moved if you focus on the more famous trenches.
Honestly if it were in the style or Gods Will Be Watching or This War Of Mine, I could see it working perfectly. You control your squad and everyday something terrible could happen to them, when you head out to secure a trench it goes into RTS X-Com mode. You move from cover to cover, trying to avoid being shelled, taking potshots at the enemy. And if you're lucky enough and RNGesus smiled at you. The trench will be taken and the enemy won't notice that its gone. Then you'll be stuck in that trench until back up arrives.
Hell you could even have a base building mechanic to make the best trench ever. Setting up traps, digging out rooms and tunnels, and setting up guard positions like DF.
But as an FPS the deary setting might become to dull to look at visually. And your only real option in the fight would be to "camp" because you don't have the right weapons to legitimately run and gun.