We assume that wealth will grow, violence will decrease, and in general life will get better for life on Earth, if only things keep going roughly as they are now. What might happen to interrupt this process?
On a Timescale of Minutes to Years:
1. Weapons of mass destruction/acute ecocide. This breaks down into three categories:
a) nukes: there are a lot of them left. This one is frustrating because it's still an existential threat but it's been around for a long time, so people have become used to it.
Once one is is used deliberately or even explodes by accident, the taboo is broken, and more will follow in much shorter order than the interval between that event and Nagasaki.
b) any other such weapons, especially biological weapons.
c) The Singularity. Superhuman general artificial intelligence would still not necessarily be intelligent enough to be evolutionarily stable, and instead of a blossoming of ultra-intelligence, could just result in ultimate ecological castastrophe.
2. Natural events that could destroy parts of the infrastructure the modern economy relies on. Earthquakes weren't a big deal when you could rebuild your bark longhouse in a few annoying hours. Not so San Francisco. 9.0 earthquakes aren't that frequent, but they occur. Similarly, we don't even know how often Carrington-level events happen because until there were electrical lines to be affected, there was no way of knowing and no reason to care. Also related to Carrington events, refer back to item 1a; the result of a nuclear high-altitude EMP would be catastrophic.
Decades to Centuries:
3. Selection against intelligence by economic development, both within and between countries. That is, idiocracy. The most fertile countries are often the most disastrous. Related: the world and technology will not change any slower than they are now (unless one of the disasters in this list occurs) yet there are people (the majority?) who appear constitutionally unable to adapt to this level of change and think in abstract terms. The modern world ironically appears to make these people regress into more and more of a fundamentalist, tribal state where they assume, correctly, that they will not be able to understand the world at all, so they cling to tribal authority.
4. Consumerism and collapse/transparency of status hierarchies, making people unhappy with otherwise stable productive lives.
a) Consumerism: it has been argued that above a certain amount (usually given as US$70,000), gains to income translate increasingly marginally to happiness. However it is increasingly impossible to escape images of houses, mates, experiences that you're not getting even with your $70,000 and your nice home, spouse and vacation. Hence this principle runs up against the human irrationality that we would rather live in a neighborhood of $100k earners and make $120k, then a neighborhood of $200k earners and make $180k.
b) This same media technology also means that increasingly, we are de facto in a world culture where there are few isolated laboratories for meme innovation. In the past, even in neighboring dictatorships, at least the more flawed dictatorship might lose on the battlefield, with the slightly better practices of the other dictatorship winning out. But what will ever fix your shitty institution now? The Mongols aren't about to overrun the DMV for being too slow. Also, increasingly we cannot preserve the independence of our multiple overlapping status hierarchies and "healthily" isolate our social spheres from one another - so your boss, or that girl that was prettier than you in high school, can make fun of your for being the president of a local organization which otherwise would've give you a nice status boost to increase your QOL.
Therefore, to avoid acute events, we should focus on continuing nuclear disarmament, start taking biology more seriously before CRISPR gives us the equivalent to Rosenbergs working with ISIS, and increase focus on AI safety. We should also try to understand how to predict and protect against Carrington solar events and similar century-frequency geological and astrophysical threats.
To avoid the longer-onset ones: no solution here would seem palatable, but otherwise we face death by a thousand cuts. To fix #3, it would seem only eugenics by licensing reproduction would work within a country, but this is abhorrent in Western politics to liberals and conservatives alike. (A certain China does come to mind and they seem to have done "okay", and by "okay" I mean "the greatest developmental triumph in human history". Despite or because of one-child?) There is also the Brave New World style solution of a big reservation or favela for all the people who can't hack it in the future economy, but in Brave New World it was just a few misfits, as opposed to all their Epsilons and Deltas.
For #4, we could adopt cultural norms about media use - while this is already happening to some degree, it takes both personal discipline and is easily eroded by non-cooperators, i.e. your co-worker who you suspect will check their email on the weekend even if they said they wouldn't. Also, solving the "tyranny of territory" would speed the diffusion of good memes, even in a connected mono-culture world. ("Tyranny of territory" is that humans have to live on the surface of the Earth so organizations from families up to government have static boundaries. Charter cities are a nice idea but fall flat as long as they are within territories held by the cartel system of mutually-recognizing violence monopolists, i.e. states. That is, I can't tell my DMV, California, that I choose to use Minnesota's DMV, because at bottom guys with guns will come and make me cooperate.)