ORIGINAL: Bil H
daz and Perturbo, yes map layer elements is the way to simulate this stuff which really all boil down to either an impediment to movement, or a multiplier to combat, or, both. I have a few ideas on what would need to be done.. let me play with it this weekend.
Here's something to consider chewing this weekend ;-).
This gets into the counter mobility additions to the existing engine.
World War I can't be mimicked in detail unless there is an increase in the type of emplacements that channel (or slow) attacks with the use of mines and barbed wire in particular.
Once the 1914-5 maneuvers stalled, the entrenchments first mirrored the effects of the "entrenched" deployments in the current game engine (nominally the kind of defensive systems that could be built during 2-days in a static defensive formation) and later transitioned to "fortified" (which the game supports only as a terrain feature rather than as a real time improvement to the entrenched deployment).
Based on recollection of the histories I've read, most post-1915 WWI battlefields on the Western and Italian fronts can be mimicked with opposing lines of "fortified" terrain, creating heavy defensive positions, and thus the terrain in which an attacking force can relatively quickly resume the defensive once it gets to the enemy's "fortified" position.
But, the effects of movement to traverse a "no man's land" riddled with mines and barbed wire may be difficult to model for real time effects.
Pace for traversing flat "No Man's Land" would be slowed to roughly the same as walking across broken terrain but would largely lack direct fire cover defensive benefits provided by broken terrain rubble. Adding a delay to that pace with counter maneuver emplacements in "broken terrain" (in existing "broken terrain" cover types, or a flat terrain riddled with artillery blast craters), may have to wait for a counter maneuver layer in the mapping engine.
Though there's an implied "combat engineer" doctrine existing for bridging and to some extent entrenchment, a more specific doctrine for combat engineer capabilities would have to be defined to allow for removal of counter maneuver emplacements, or given a game duration which allows it, creation of those emplacements or improving "entrenched" to "fortified" during the scenario.