We can all moan about our own countries and yearn about how to make things better (I'm just about to have a major moan about the UK government on the brexit thread, as it happens), but as this is about America, and as I see you've already mentioned guns, I'd add:

Never again try to undermine a foreign government simply because its allies are not yours, or because you think they're a bit too socialist;

Emasculate the pro-Israel lobby and stop unconditionally supporting the Israeli regime;

Recognise that it's untenable that a country with 4% of the world's population should be producing 18% of its greenhouse gas emissions.

(I could mention abortion, the death penalty and such stuff but I'll avoid the temptation of turning this into a litany)

The US is indeed the world's largest democracy, which is good. Almost half of your voting electorate voted for a man who is your biggest threat to democracy. That is, in some regards, more worrying than that man himself. We have a very similar problem here.