The ability to put barbed wire on metal fence posts.
We can build metal posts, but only as part of the chainlink fence construction. Barbed wire can not go on it.
I suggest adding the ability to build metal posts so we can attach other fences to them. This could be a slightly stronger version of the other fences.
And to add a third tier to the chain link fence construction that adds barbed wire to the top. Making it dangerous to climb while still having the strength of a chainlink fence.