WW1 is ripe for gameplay. For one, read up about the pilots during the war. Its facinating stuff. Originally, before they perfected machine guns that could shoot through the propellers, air to air combat was done- no joke- with the pilots holding pistols or rifles and trying to shoot other pilots. If you wanted to drop a bomb, you wingman had a bomb in his lap and literally just dropped it out the side of the plane. Later the interrupter gear was developed, allowing the planes to have forward firing machine guns. Of course, the main point of planes were reconnaissance. Pilots lived an almost nomad-like life, landing in the country side, hiding the plane, finding the nearest friendlies, and refuelling before returning to base.
Now tell me that wouldn't be an awesome game. Part Beyond Good and Evil (photography), part flight simulator, part first person shooter, part stealth (landing behind enemy lines and avoiding capture).
Additionally, there was some fighting done outside trenches obviuosly. Most soldiers participated in a raid at least once, but some commanders did nothing but raid the enemy trenches at night, which would be fun. To give you an idea of what that gameplay would be like, consider: if you party met an enemy party in no man's land, you couldnt use guns. The noise and flash would send every machine gunner on both sides of the line to light your area up. You had to kill the entire enemy party with you bare hands.