As for the Neptune idea, assuming 200+ years of technological progress in chemistry and physics, isn't there something that could be remotely viable about the idea? (Not fixating on Neptune in particular, but as in using materials from other planets.)
Restoring the ozone layer by dispersing ozone could be a useful idea, but all it requires of material is oxygen, which we have plenty of right here. Neptune has nothing extra we need. The sun turns oxygen into ozone by the gigaton, today. To really address the problem, you would need to remove the fluorine compounds that consume the ozone. Nobody seems to have a better idea than not to vent more.
The space elevator notion relies on discovering a material strong enough to build it of. Other schemes seem possible with existing materials.
Neptune has helium-3 that might someday be useful as fuel for aneutronic fusion, if we can get that to work. Hardly anybody is trying. Getting to Neptune and collecting it seems even harder than getting aneutronic fusion working.
> A skyhook differs from a geostationary orbit space elevator in that a skyhook would be much shorter and would not come in contact with the surface of the Earth. A skyhook would require a suborbital launch vehicle to reach its lower end, while a space elevator would not.
https://en.wikipedia.org/wiki/Skyhook_(structure)?oldformat=...
I.e., Blue Origin's pogo stick, or Virgin Atlantic's tour bus, would suffice. The requirements on such a vehicle are massively less than for an orbital launcher.