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Solar powered 3D printers that use the carbon in CO2 to make stuff.

That sounds crazy, but why not actually? Trees do it. CO2->O2+C where the C is used for say nanotube production sounds pretty awesome.

...or, why not some kind of Lego (?) nano bricks that can easily be ordered/reordered?

Greatest would be if the printers could print (replicate) themselves. A new life form :)

There's already a CNC printer that can replicate itself. You just have to buy the raw material. For example, see this

Pretty sure we already have those... they're called "trees"