Add some in-game modding by giving Engineers a class skill of "Fortification Design". The new skill lets those engineers stack a variety of hollow polygonal forms and a few details (think ladder, parapet, door, window) to create a fortification on the fly. Think the Spore(tm) building creater with its multiple building parts and details but simplified for FPS. Obviously not on the complexity scale of Spore (before the naysayers comment). Potentially redundant to building walls, but would be 'fun' in letting the engineers come up with something novel and strategic. E.g. build a nifty stronghold at a chokepoint or entrance.
