Consequence of Sound recently compiled a list and gave odds on a host of long-rumored, mythical, likely and unlikely albums -- all long overdue. It's a pretty good read; I consider myself a Springsteenophile but I didn't know about the "electric version" of Nebraska!
Here's a few more. What else do you think should see the light of day?
Bob Dylan - Concert recording from Toronto April 1980. This was from the gospel years, and apparently an entire album (and possible DVD) was mixed by Daniel Lanois; the audio has circulated in bootleg form for years. It sounds amazing on my cassette copy.
Charlie Rich - Demos. The name of my show, "Feel Like Going Home," is taken from a Charlie Rich song (and a Peter Guralnick book). The song was recorded at least three times by the Silver Fox, but the greatest is a simple piano demo which can be found on the collection of the same name. That leads me to speculate that more such demos are in a vault somewhere.
Patty Griffin - Silver Bell. Her third album was completed but never released by A&M before she was dropped. It can be had on the Internets, but still hasn't been officially released.
The Wrens - Followup to Meadowlands. Yes, it really is rumored to come out this year, but I'll believe it when I have it in my anxious hands.