I think for minor things in different cases for each map.
1. A small sandbag thing for most maps.
2. A trenchline (Needs to mess with meshes somehow) for Brits in the NA desert
3. A foxhole (Same thing), also map specific - Like in the Bulge
4. Using plungers and detoners with xpack could come under this catergory.
5. As could a sniper tree stop for any tree - It would no longer rely on the tree being a vehicle, but the ability to position it, although I foresee this failing due to an ability to dump it ANYWHERE
6. Barbed wiring and ability to cut through THOSE barbed wires.
7. The variety of mines the Germans had, and the Japanese use of artillery for IEDs. The kind that could blow up a tank in one and even toss it over.
What would restrict building stuff in stupid locations is
1. They can ALL be destroyed
2. They take a long time, and you will be sniped doing that close to the enemy
But ability to do so will definitely make even more dynamic gameplay imo.