My list is quite different than the ones currently in the thread.
The boring ones:
Creating a vaccine or other cloaking to make humans invisible to ticks & mosquitoes. A separate project would be to do the same for parasites.
Enacting strict pollution/carbon limits and mandatory circular economy everywhere in the world.
Researching, trialing and Enacting a sustainable post-capitalist system everywhere in the world.
Developing solar energy until covering global energy demands, including a power network that can transport energy from the sunny side and/or orbit everywhere.
The slightly more ambitious:
Establish self-sustainable colonies living on off-earth resources, most probably also situated off-earth.
Create a Dyson swarm with enough energy output for in-system exploration, mining, colonisation, and terraforming.
Perfect matter replicators.
I have some other ideas as well, but those would be a start.