In a post-cataclysm world, only the most heavily fortified of settlements will be viable in the long term. It would be nice to have the addition of the ability to construct additional fortifications than what is presently available.
Pillboxes:
Currently you can make SOMETHING like this with walls on the sides and sandbag half-walls in the front, but I was thinking it might be nice to have a concrete or metal wall section that obstructs movement but not firing, like a firing slit. This would make pillboxes more viable than having only a wimpy sandbag wall between you and that skeletal juggernaut.
Electric Fences:
This is probably something that would have to come after power grids and appliances are working, but the ability to electrify a chain-link or barbed wire fence would be cool. Have a solar array or generator and hook it up with jumper cables to a transformer built adjacent to the fence section, and all contiguous fence sections get electrified.
Moats:
Of course, everyone knows about the humble moat. Being able to dig a trench around your base and fill it with a liquid to slow down attackers would be nice. Or even better, fill it with something that will outright kill them. Why fill it with just water when you could throw in some man-eating SEWERANHA? Or better yet, forget water, fill it with concentrated acid to dissolve enemies, or gasoline/diesel/napalm to create a wall of fire in a dire emergency.
Security Gate/Portcullis
The regular wooden gate is a bit limited. Why bother building concrete walls if the gate is made out of WOOD? The ability to construct a large opaque plate steel security gate or portcullis would be an excellent addition to fill this gap. Might need an engine, electric motor, or gearing & pulley system to lift.
Foxholes/Defensive Positions/Trenches
The humble shallow pit would be nice if it added some additional bonuses to defense or reduction of a chance for an enemy outside it to hit with a ranged attack. Combined with sandbag half-walls, it would make a nice defensive position. Or if you wanted to make a series of connected foxholes, it would become a trench network.
These are just a few examples of some of the many various defensive constructions which could potentially be made available.