A little bit of speculation:

Last month, New Scientist had a fascinating article about "superatoms", clusters of atoms which start acting like different elements altogether, due to odd synergies of the clusters' valence electrons. "We can take one element and have it mimic several different elements in the periodic table". Simple aluminum can act like a higly-reactive halide, or like a noble gas, for instance.

At the time, this seemed like a cool novelty, but not immensely worldchanging, especially since it's still rather speculative. However, if you combine this idea with nanotech atom-by-atom fabrication (also speculative, but bound to happen at some point in the future), could you perhaps make an eco-fabricator that makes anything you want, all out of nitrogen? Or maybe all out of carbon? (Some ubiquitous, mostly-harmless element.) Instead of requiring inputs of metals, nonmetals, organics, inorganics, etc., perhaps the ultimate in nano-fab would be to make everything out of one element, clustered into superatoms where necessary to take on the properties of a metal, inert gas, or other element.

It's utterly pie-in-the-sky, but maybe something to remember in coming decades.