My personal favourite is LISA - low frequency gravitational wave detector, a close loop laser interferometer, with a nominal 5 million km baseline (yes I said Million). Way cool. I'll be saying a lot more about LISA and its proposed descendants. LISA is joint with ESA, which is also interesting on so many ways.
Con-X was another, next generation x-ray observatory, optimised for spectroscopy, it is in trouble, but on life support for now at least. I wish them well. More on that late too.
SNAP^H^H^H^H^H JDEM is another, a wide field camera, barely sub-Hubble, to look for type Ia supernovae and try to measure the equation of state of the dark energy (yes, more on that later).
Beyond that, it gets ambitious - cosmic microwave background polarization measurements, x-ray interferometers, detection of gravitational radiation from the Big Bang. Fun stuff, with 15-25 year planning horizons. All now deferred, postponed or possibly in the trash can of power point plans everwhere. Hopefully most will get done, in some for, eventually, and before a generation of expertise is lost waiting for the next mission.