I suggest adding an Engineers unit in the Great War. They would be able to deploy/construct different defensive placements in the terrain like barbed wire and new ones like MG nests. Would be cool to have those..
I think it would be an interesting idea to add in swamps or deep wet dirt in certain missions and terrains. It will decrease the movement speed and fire accuracy of the units placed there, over time will lead to death of some soldiers and will increase the chance of the whole squad to panic and flee if attacked. I think not just about swamps in the lower grounds on some maps, but about areas of swampy dirty craters that appear around the areas which are heavily attacked by artillery over some period of time. Let’s say the further is the artillery from the place it shoots the less damage this place gets but that means the more shells fall and explode around it and create an area with big holes with dirt and water. Ofcourse in campaigns that takes time in fall or spring.