More ways to acquire/reclaim metal, and not making everything cost metal.

[ShadowDragon8685]

Alternatively, you could build mineshafts and quarries, into which colonists disappear for an amount of time, and then out pops a colonist and metal/rock!

[Sundaysmile]

Alternatively, you could build mineshafts and quarries, into which colonists disappear for an amount of time, and then out pops a colonist and metal/rock!

That's actually not a bad idea, has a kinda old school Warcraft RTS flavour to it.

At least that way the dev doesn't have to worry about Z axis's, and it could still keep the Mining Skill relevant even after EVERYTHING on the map has been bulldozed lol. 

This carries my support.