I think the big problem with a lot of people trying to make a world war 1 game is that they always find that making a game about trench warfare is really difficult and not as fun as a game should be, but so many people forget the Eastern front in WW1 and how it was much more conventional styles of warfare between the Germans and Austrians vs the Russians. I really enjoyed a little known FPS by the name of Darkest of Days, and it features time traveling agents that are going back to a couple of famous wars to try and rescue the ancestors of important people because the timeline has gone wonky. One of the wars was WW1, and you play through the Battle of Tannenberg. Trench warfare was there, but not much of it.
I feel that if more games that attempted to be about World War 1 focused on the Eastern front then the idea of making a World War 1 game would be less of a daunting task and more of a successful endeavor.
As the producer for rising storm, I believe the reasons that TWI handed the expansion/mod (some semantic difficulty here as the decision about paid or not will come when it is nearer release) to a team of modders are two:
1) Gives them a chance to support the modding community that they came from when they won MSU all those years ago. 2) Frees up their devs to work on something else (can't say what)