I think it would be addressing climate change.

I would be happy if the global funds went into removing carbon emissions and oil dependency

The Apollo moon landings were like real-life sci fi. The culmination of the space race between super powers doing stuff from way into the future. It had been building for years and Apollo 11 the crescendo.

We'd only just got a land line and we watched 2 guys walking on damned moon on B&W tv!

Modern tech has to an extent inured us to amazing stuff. We have the attention span of gnats driven by culture and media. Someone does something clever with a computer a phone ,software or whatever , we go, that's clever and tomorrow its old news and we're excited by something else.

We now expect amazing.

Everything is wrapped in hyperbole and there are so many others things for people to do and see now.

It is hard to imagine anything that would make the world stop and go wow! And that wow lasted for quite some time.

Successful nuke fusion , faster than or near light speed travel. Detection of aliens or alien signals of sorts. Meeting a tourist from the future or past perhaps.

I think if ITER works it will be a similar watershed moment for a lot of people. The implications would be significant.

But I don't think there can be any technological achievement which resonants with millennia of human wonderings quite like the moon landing did. That was something you could explain to a Roman and they'd understand the significance of.

If I could postulate anything without feasibility being important I'd say a step change in human life spans. Something like "immortality" say another healthy hundred years arriving all at once.

Imho, a permanent base on the Moon. MUCH more likely to happen than humans/permanent base on Mars.