The power to choose, just before dying, to come back to life as a healthy 30 y/o.

But you probably meant only selfless acts allowed, hence the "sacrifice" verbiage. In that case, instantaneous worldwide post-scarcity society. That'd address most of the world's ills, eventually. Being ultra-rich means nothing in post-scarcity; not needing Middle-East oil and African mineral resources would eliminate most international meddling in those locations. While it wouldn't immediately address climate change or cure all diseases, it'd mean enough food and energy for everyone, and it'd give us the means to accomplish these.

If so many of us weren't spending all our time working just to feed, clothe, and house ourselves, we could accomplish much as a species.