How about a set containing the Complete Columbia/Sony & RCA recordings of Charles Ives?
We were discussing some of this music recently when we were discussing the Gregg Smith Singers' recordings of Ives' choral music, the best of which has never been issued in any digital format. The set would also include Ormandy's superb recording of the Holidays Symphony for RCA, which was only available in digital format as a CD-R through arkivmusic.com. Naturally, it would also include all of the great Columbia/Sony recordings by Bernstein, Stokowski, and MTT.
It's unlikely to happen because it's probably not financially viable. But one can still hope.