It turns out, humans don't just mindlessly reproduce. When we have access to birth control, when society takes care of old people, and when women are educated, we have the opposite problem: birthrates are too low to replace ourselves.
It's funny, just twenty years ago, this doom scenario wasn't even on my radar: the whole world voluntarily having fewer kids, leading to a collapse of growth-based economies, and a glut of old people without enough young people to take care of them. But that's probably what's going to happen, at the same time as climate change, and whatever crazy stuff happens with AI and biotech.
Into this mess, I want to throw one idea: psycho-geography. Basically, some towns, cities, and neighborhoods, will be more mentally healthy than others, and those places will become magnets for the dwindling population. Meanwhile, the less mentally healthy places will become nasty, and eventually abandoned.
I mean, this is happening already, but now the migrations are more about money, which is sometimes the opposite of mental health. In a post-capitalist economy, developed nations will be preventing famine through social services that you can get almost anywhere. So migrations will be more about culture, or other non-financial measures of quality of life.
Looking farther ahead, the most successful localities could define the next age, maybe in different ways, as nation-states fade.