I would want:
1) Meddle by Pink Floyd, although Animals would certainly be welcome.
2) Graceland, by Paul Simon (not prog-rock, but would make a good MC release).
3) I think several of the albums by Renaissance would make nice MC releases.
Close to the Edge, Thick as a Brick, and the entire Quadrophenia, as have been previously discussed. I just noticed somebody mentioned Eno, I would be very into Taking Tiger Mountain By Strategy...
I also think Blues for Allah, by the Grateful Dead, would be interesting. I wonder how Live Dead would sound in MC, although it probably wasn't recorded in enough tracks to make an effective MC mix.