WW1 isn't a nice context. It's not what I call a military RP entirely, rather just an RP that is based around the great tragedies of a past generation. And most people I know wouldn't want to sign up for an RP like that. It sounds really shocking, and Rare, I do agree- it would be great, but for me, it would be a lot of micromanagement to do, and there is a lot of theatres, and factors that I couldn't be responsible for to implement to have the RP to a high standard. 

Another fact is that to truly understand it's impacts is to be European (I assume you're British), or even live in Europe/Canada/Australia. Surprisingly, it doesn't sell well to Americans in relation to the concept- it is horrifying, not being a war really, but the pissing away of millions of lives for the needs of a few Imperial ambitions, from the Tommies to the German troops. What did we have at the end? Hitler, the crushing of the German Empire, the consolidation of an existing British Empire, and millions of soldiers in graves spanning from France to Belgium- not the defeat of some dictator or liberation of a country. I wouldn't consider it a military RP anymore- it's just a group of individuals trying to survive the trenches, and not get shot/gassed/bombed/sniped etc..In regards to the American comment- people can prove me wrong, and seriously, I'd be happy to hear it. But it's still surprising on the whole, the American knowledge of what happened BEFORE 1917, when the Americans entered the war. The Somme, Verdun and Marne are forgotten- they are events that in the context of history of the war, and without them, and the whole story of your soldier, you'd never make sense of it.

I could do a WW1 RP in the context of it being not a military RP, but again, the focus of a set group of individuals, perhaps not even on the same side, put into a situation beyond human comprehension, with conscription and the like. The war as merely a backdrop- it isn't a focus, it is a mere background noise that fuses the characters to one another, and it has arguably, quite a lot outside of it, perhaps families, friends, politics (though this is through News and not played characters). It'd need to be a very sensitive approach- and is why I stray from it, whilst modern/contemporary is a little easier to complete- though I'd still say you are right, a WW1 RP if done right can be done. 

If anything, other periods of time I'd say are underdone, but need to be done are (dates are not specific):
Warsaw Uprising (1944)
Telemark Raid (1942)
Falklands War (1982)
Boer War (1902)