How about a campaign for building any developed defensive position.
This idea was inspired by two current discussions:
Build a diorama of any size depicting any form of developed defensive fortification. This could be a developed bunker, flak emplacement, flak tower, fortification, machine gun emplacement, tank berm, tank trap, or section of trench. Significant effort was expended to develop the position for long term use.
Maybe call it Digging in or Holding the Line.
For inspiration, please see German Field Fortifications on the Eastern Front.