The Solar Disk concept
The NASA Solar Disk idea would be crazy expensive, but it would also usher in a new age of clean technology for the entire planet. An enormous project like this might take a few centuries to pay off, which is one of the major criticism of large-scale space solar. But if we don't invest in such groundbreaking ideas, will there even be a hospitable planet for us to live on a few centuries down the line? That is the real question. No offense, Hamlet.