For almost every 1st world country outside of the US (who can/will pull the immigration lever to supplement their working age population) and China/Russia (who are proper fucked because of their economic structure and unwillingness to embrace immigration), the next 30 years will be a very risky time as they go through an unprecedented societal experiment.
Can Europe, Japan, S Korea, and Australia maintain their standards of living despite dramatic declines in working age population and economic activity? That’s something never before seen in human history.
If that’s not possible, can those same countries somehow turnaround the population decline to at least eventually achieve population stability? Again, never before seen in human history.
There will be a lot it things that can be done in the short term (extending what is defined as “working age”, fully embracing automation and robotics to replace human workers) to stem the tide, but those 2 questions will really determine the next phase of humanity.
One thing that is interesting to me is that of all the books/articles I have read on this topic, none of them predict that the countries who currently project for continued population growth (S America, Africa, India) will rise to the top of world powers as other countries populations collapse.