I'd pay good money for a well made WWI FPS, and I'm not even a big fan of FPS games. People say it wouldn't work because just sitting in a trench getting diseases is boring, and yes I agree with that. However, WWII shooters usually put you right into the interesting parts, skipping the boring, and I don't see why a WWI shooter couldn't do the same. And as far as I know, the eastern front was more mobile, so you could have other than trench levels as well. I think it should be more on the realistic end of the spectrum, though.
WWI would make for a really interesting setting, I think. Battle of the Somme, anyone?