i've always thought that a COD game set in WW1 would be a good option for either Activision or Treyarch to make (particularly because horse-riding was introduced in Black Ops II, so there's precedent for its inclusion on a larger-scale in a WW1 game)
getting to the point, what would need to be done for a WW1 FPS to work would be to just focus campaign missions on the actual conflicts. i mean, when people says that WW1 is a too entrenched and slow war to make a game out of, they're assuming that the full experience of the trenches would be the bulk of the game. it wouldn't be. WW2 may have been more fast-paced than WW1 (despite actually being longer than the former) but COD, for instance, still focused on actual engagements and not the time inbetween. Black Ops focused on the actual secret operations of the story and not on the careful planning inbetween (the closest thing to that is meeting with Kennedy when he authorizes the assassination of Dragovich).
anyway, for the setting and gameplay, the player character and immediate allies would just need to be elite soldiers rather than common grunts so that they realistically spend more time rushing other trenches rather than waiting and popping off a shot every once in a while, though hiding in trenches would have to be a gameplay mechanic in order to get closer to a machine gun nest, for instance. the trenches could also have more strategic uses in a given campaign mission (i'm not gonna touch on a multiplayer mode) such as tricking an enemy tank into falling into the trench and getting stuck so that you can run up and destroy it via a grenade down the hatch (kinda like how you can stick a grenade on a tank in Halo). speaking of tanks, using those in at least a few levels would be a must, perhaps along with airplanes. a German portion of the campaign could very well have the player as a member of Jasta-11 and fighting alongside the Red Baron himself, with one of the more (intended) emotional parts of the game being Richtofen's death at the Somme. there could also be an easter egg where, if you hover the cursor/sights over a German soldier running past you (in a German campaign) it says "Pvt. A. Hitler", with him playing no role in the story.
in fact, there could very well be two WW1 games like this, one where you play as the Allies and one as the Central Powers, both being the same game but each having a unique campaign and some special multiplayer maps, though multiplayer would work between both games (as opposed to only with their own game). kinda like how Nintendo always releases two titles in the main Pokemon series, with both being identical except for a few differences of what you can get in-game and occasionally some differing story elements. the Allies game would, of course, have the player as the British, French, and Russians (and maybe the Serbians to mix things up a bit, depending) while the CP game would have them as Germany, Austria, or the Ottomans (duh). notably, the US could/should be excluded because its honestly overused and, in the context of WW1, they came to the war very late, whereas there were still three more years in WW2 when they joined