You know, with a metric fucktonne of WWII games out there, I'm a little surprised there aren't more decent, well-known WWI games out. Like, the only strictly WWI game I've played or even heard of is Red Baron 3D. Admittedly, Red Baron 3D was pretty damned awesome.
With all the "real is brown" going on, wouldn't some bright bulb out there say, "What's the most brown a war has ever gotten? Oh! The mud-covered trench warfare of WWI!" Grime, dust, mud, primitive tanks, shitty French machine guns that look like they were designed by a child with Down's, fucking mustard gas attacks...the list goes on. If nothing else, you could make a game like SpecOps: The Line, only set in WWI, and likely add more horror to it because that shit was fucking grueling and painful and brutal as hell.