Human Longevity.
150,000 people die every day, and 2/3 of them die of age related diseases. The developed world and China are racing towards a demographic nightmare where fewer and fewer people are left to take care of the elderly. The older someone becomes, the more of a burden they become on the young. As people get older they begin to develop the diseases of old age and have an increasingly pain-filled life. People are forced to save for a future where they have enough to survive for a few years and then pay to have someone take care of them, instead of doing what they actually want to do.
Why are we not all collectively trying to solve this problem? Dying of age related diseases is as natural as dying from malaria or childbirth, yet we try to solve those issues and pretend age related diseases are inevitable. COVID largely shut the world down because the largest risk factor for complications is how old someone is. The gains in productivity alone should make human longevity the number one priority for every government.
I also think it would solve climate change because if you expected to live much longer, the obvious problems are no longer just future generations problems.