Not saying I endorse these fully, certainly not to the extent of them being the “whole plot” and making other considerations irrelevant, but I think they both contain enough of a kernel of truth to be worth mentioning:
1) While not quite an existential threat, climate change seems posed to cause death and suffering on a considerable, perhaps unprecedented, scale within this century, and will likely also act as a “badness multiplier”, making pre-existing issues like disease, political instability and international conflicts worse. Absent technological advances to offset these problems, the destruction of arable land and increasing scarcity of drinking water will likely increase zero-sum competition and make mutually beneficial cooperation more difficult.
2) More speculatively: due to the interconnectedness of the modern world, our increased technological capabilities, and the sheer speed of technological, cultural and political change, the world is becoming more complex in a way that makes it increasingly hard to accurately understand and act rationally in—the “causal graphs” in which people are embedded are becoming both larger and denser (both upstream and downstream of any one actor), and unpredictable, non-linear interaction between distant nodes more often have an outsized effect—black swans are becoming both larger and more common. The central plot is that everybody has lost the plot, and we might not be cognitively equipped to recover it.