Hoverboards immediately sprang to my mind, other than that…
Oh I dunno….. the perfection of nuclear fission or some other viable big energy, no/low emission power source.
Or maybe a definitive social/economic policy the whole world could prosper from.
But then again, a cheap, easy to use, personal jetpack would be good.
Damn, I just realised a problem… little chance of 1 and 4 without 2… bum 🙁