Here's how I think it could be done, a combination of scripted scenes, but mostly procedurally generated stuff.
The game would start with a pretty standard tutorial, learning how to use rifles, grenades etc. But emphasizing the rigid social structure at the time and the colonial jingoism at the start "tut, tut the war will be over by christmas and we won't have given the kraut a sufficient walloping". This of course would all change throughout the game. You'd probably be a reserve officer or something, fresh faced and straight out of school.
The first mission would be heavily scripted, you getting ready to go over the top for a charge. The atmosphere would be incredibly tense, from the music, the soldiers muttering and the sound of artillery fire. You'd feel scared of going over the top.
Then you'd leap over and run through no man's land while machine guns and artillery obliterate everyone around you horrifically, not a cinematic but rather if you bother to turn to the left you'll see a couple of men ripped to pieces by machine gun fire. Eventually a shell would land near you and you'd pass out.
You'd awake in hospital for a brief moment before it blacks out.
Now the bulk of the game would be about a single large area, your lines (+ no man's land and the germans) and perhaps the nearest village you billet at while on rest.
The game would a fps with rpg elements like STALKER, there'd be NPCs from officers who'd issue you orders, to supply clerks or medical officers who ask you favours (like bringing them so much of x) and just soldiers with great accents and funny senses of humour.
At your lines you'd be able to walk around reasonably safely, you'd have to be careful and crouch through sniper trouble spots, wade through the muck with rats and every now and then you'd hear the crump of artillery and have to seek cover or hit the bottom of the trench.
No man's land would be ever changing and initially very frightening, but eventually you'd begin to learn safe spots, good sniper vantages and the quickest routes as you went out on night patrols. It would change each sometimes not by much, at other times huge differences. Every now and then a flare would go up illuminating the whole area before fading leaving it darker then it was before until your eye readjust.
Missions would consist of repelling German attacks (sometimes fighting in the trenches), assisting artillery and mortars, sniping the German lines, scouting no man's land, ambushing patrols in no man's land, retrieving wounded and dead, and off course attacking the enemie's line.
You wouldn't go over the top often in an all out assault, but you'd dread every time, sometimes you'd be successful and hold a trench, until your relieved or ordered to pull out, but inevitably you'd lose it, and you may even get badly wounded/die. When you got killed in game, you'd later wake up in hospital being told you'd been bravely rescued and dragged back. If this happened too many times the game would end with you being sent home crippled or actually dieing and you'd have to reload a save or checkpoint.
After every "big show" npcs you'd gotten to know might be crippled/dead, this would be random and depend on what happened in the fight. You might hate certain officers and almost be happy when they bought the farm or change your opinion about them when they do something amazingly heroic. A NPC you like might make you think of doing something risky in the hope of saving them. A wounded NPC might right you letters from hospital or from home if they are crippled.
As the war progressed the nature of combat would adapt, you'd eventually have plane and tank support.
Weapon wise you'd be able to carry three type of weapon:
Primary:
Simple bolt action rifle (both English and German) and would have the advantage of a bayonet
Scoped bolt action rifle
Lewis Gun (later in the game)
Double barreled shotgun (perhaps as award for side questions)
Pump action shot gun (side quest)
Anti-tank rifle (agains later in game)
BAR rifle (later)
Prototype submachine guns (sidequest + later in game)
Secondary
Webley revolver
Mauser semi-automatic or a Luger(captured from Germans)
Colt 1911 (later in war)
Melee:
Bayonet
Bowie knife (captured from germans)
Cosh (steel rod rapped in leather favoured by English)
Improvised trench clubs
Officer's swords
Sharpened shovels
and more stuff
Your pistol and melee could be used together at times. You would also carry a couple of grenades.
A heavy element of combat would be the melee side, either a brutal scuffle in the confined space of a trench or silent killing in the dark of no man's land. You'd be able to perform melee attack like elbowing and kicking like in condemned also.
The gun play wouldn't necessarily be slow paced, I mean Call of Juarez make even older and slower weapons quite exciting. Player would have to get to trying to be accurate and maintain cover before closing in violently with bayonet or shovel.
I should also mention gas and disease. If pushed though gas would create in game, impassible spots in no man's land and another thing you have to worry about on your lines, you'd have to run get a crude gas mask or something.
Disease, could be mentioned quite often (by medical officers, NCOs etc), but would be harder to implement in the actual play. At most things like trench foot and respiratory infection could be implemented as restrictions to running speed, health etc. But anything worse might have to skip by your character, for the shake of maintaining the story.
I've also been thinking that environment having effects on your clothing could be implemented quite well. You start each day with a pristine (well at least cleaner) seat of clothes, each time you hit the dirt and land in mud, the clothing gets dirty, can get torn when going through wire and bloodstained in combat.
Sample from game:
Imagine this, its a few hours before dawn but very illuminated by the full moon, you and two other soldiers crawl up the side of a crater, you see German's placing heavy rolls of wire over the edge. The three of you pick a target and fire simultaneously before firing off the more rounds rapidly and throwing crude grenades, this met with screams of anguish and surprise.
Without a moment to appreciate your work you fling youself back into the crater as machine gun fire rakes the top. Scrambling down the slop and wading through the water of the shell crater, you start to run stumblingly as you hear the "fwoop" of a mortar. You get out of the crater just as an explosion lands nearby.
You get separated from your two comrades who are hightailing it back to lines, when you dive into another artillery crater, this one filled with corpses from a previous attack. As you peer over the edge of the crater prone, a noise behind you makes you start to turn. One of the corpses isn't dead (rather he was hiding) and his hands are wrapped around your throat. Your rifle gets knocked out of your hands. You draw your revolver and pull the trigger, but with all the water and mud its doesn't fire. Rapidly, as the screen goes back, you click to slam the butt into his head. His fingers loosen and you realize once again you're the only living man in this crater.
That would be something to talk about around the water cooler.
