The most "immediate" threat is Donald J. Trump. Giving one of the most reckless, narcissistic sociopaths the world has known the power of the US presidency is like playing with fire at the gas station. He could easily start a nuclear war, a conventional, or a civil war. He could destroy an economy, indeed the entire world economy. His denial of ecological peril is wrecking havoc on our economy. His maniacal fumbling with our government is destroying faith in not just our democracy, but also our faith in democracy as an idea.
Sure the world's economy and environment are endangered, but DJT is the best playing with matches out by the gas pumps.
Long range, and if we survive Trump, I agree with the comments about trying to raise the standard of living around the world and overpopulation. Maybe we need a good old-fashioned epidemic! But we also need 6 billion people to give up their dream of owning a single family McMansion, driving an SUV, going on exotic vacations, eating steak or dessert every night, etc. America has now exported consumerism to almost every corner of the globe. It will only get worse if everyone wants to be like us and during the last 20 years we've even seen that aspiration growing in China.