HoI3:
WW1-Mod
There is also a german Modding group modding a WWI mod at this time. They have no version out but work also already 1/2 year on it.
My final goal with my AHOI-Mod is also to be able to play from ~1900-1970.
I made a complete new techtree for that already(1001 new techs, giving around the same amount per techs/year as HoI3) and a first version of new production-ai/research-ai to cover that timeframe. The current version covers only WW2 though. See my sig for more infos.
I personally think so far, that the game has everything to model also WW1. It is just very work intensive to get it done.
Then we have already much more polished Mods for Darkest Hour wich has a WW1 scenario built in already..:
Mod1914-for-Darkest-Hour -Is an update of the older HoI2 Mod.
Arms-Armistice-and-Revolutions-Announcements -Enables DH WW1 sceanrio to be connected with its WW2 sceanrio.
And the new project
The-Grand-Campaign-(AAR+NWO)
(wich lets you play from 1914-1964, as it combines DH and the Mods AAR+NWO)
So "Paradox" did already made a WWI game to some extend.

Ask the DH coders if you miss some urgent WWI feature. They might add it in an patch maybe..
Cheers,
Chromos