evolved Laser Interferometer Space Antenna / New Gravitational Wave Observatory

(It does not sound like it has been funded yet)

Would be the largest instrument ever built by humanity. An enormous space-based gravitational wave detector that will measure distortions of space-time, revolutionize astronomy, our understanding of physics, and the past and future of our cosmos

It would be the largest laser interferometer ever constructed: Three spacecraft, separated by a few million kilometers, sending laser light back and forth. A LISA-class mission like eLISA or NGO would enable us to listen to the sound of the Universe.