1) A functional healthcare system like any of the other half a dozen good alternative approaches in other developed nations.
2) For the government to stop inflating the cost of education by enabling universities to raise their prices outside of market forces (eg income growth) via perpetually greater government loan backing. Compare the median income vs median tuition costs, just from the late 1990s vs today, the government made that imbalance possible. For a century the US had very reasonably priced universities (40 years ago you could pay for an Ivy League education with a part-time job), that ended in the very early 2000s when the government inflation run took off (exactly in tandem with healthcare inflation and dollar debasement under the Bush Administration, which you can see represented in the commodity bubbles, housing, gold and the dollar GDP of every other nation soaring simultaneously from the year 2001 to 2008).
3) Significantly improved cities. More people from rural America need to be encouraged to move into urban centers, to resolve the opportunity & quality of life gap (such as falling life expectancy in rural US) that is widening between the two. That includes improving infrastructure, transportation, and available housing supply (entailing, among other things, pushing back against overly aggressive zoning laws used to restrict housing). Urban improvement / renewal, would benefit basically every demographic, including poor black communities that are being persistently hammered by extreme violence.
A possible #4 candidate, would be to continue ending the war on drugs, shifting to treating addiction as a health matter and decriminalizing or legalizing all drugs. Sentencing reform and police reform would also go with that item.
Doing these things would radically improve the circumstances for America's youth as a whole.