These are my ideas for the different stories- A French trench soldier A British tank gunner A US fighter pilot A German Zeppelin gunner An Austro-Hungarian MGunner An Italian sharpshooter An ANZAC at Gallipolli An Ottoman at Suez A Canadian trench raider (linked to the first French trenchman) A German soldier in the trenches. That covers most of it, right?
Don't have so many guys, you'll have way too much to do. But if you do, I'd do a series, with like 2-3 perspectives per book. Then you tie them all together somehow, like a post-war memorial or something.