Over the years, due to personnel changes, record label resistance, or any myriad of other reasons there have been albums which were either never finished, or finished but never released-- at least not in their original form. These are some of my favorite rock and roll album "what ifs?":
Beach Boys Smile (1966): While Brian Wilson did finally realize his musical vision in 2004, and over the years the Beach Boys did release various tracks that were supposed to be on this-- "teenage symphony for God." I do wonder how the Beach Boys would stack up next to the Beatles today had this musical vision been realized at the time of Wilson's initial creative inception.
REO Speedwagon Ridin' the Storm Out (1973): The album was almost finished, the cover art had even been completed when lead vocalist Kevin Cronin opted to leave the band to pursue a solo career. All tracks were re-recorded with new lead vocalist, Mike Murphy, and Cronin's face was airbrushed out of the cover art and replaced with Murphy's face. One track from the original Kevin Cronin sessions was released on REO's A Decade of Rock & Roll: 1970-1980 (Son of a Poor Man), but as of now the rest of the material from the original Kevin Cronin sessions remains unreleased. Incidentally 3 years and 3 albums later, Mike Murphy left the band and was replaced by... Kevin Cronin, whose "successful" solo career had never materialized.
Toto Isolation (1985): Bobby Kimball has said in interviews this album was almost done when he was fired for drug abuse in 1984. Fergie Frederiksen was hired to replace Kimball and the album was re-recorded with a bit of a different track listing. As much as I love Frederiksen's soaring falsetto, I'd still love to hear what Isolation would have sounded like had Kimball never been fired. A few of the tracks that were recorded for the original Kimball-ized version of Isolation did show up on Toto's 20th anniversary rarities collection XX, it has been rumored from time to time that with Kimball back in the band (after a 14 year "vacation") the band may actually release Isolation as it was originally intended-- with Bobby Kimball on lead vocals.
Chicago Stone of Sisyphus (1994)*: Unlike the other "unreleased" albums mentioned, bootleg copies of SoS were leaked out to fans and have been circulating for years. So I have actually heard it in its entirety. This is easily Chicago's best material since the departure of Peter Cetera-- possibly even their best since the untimely passing of Terry Kath in 1978. The record label absolutely hated it and it was inevitably shelved. For the full story, you can read my review of Stone of Sisyphus. This album has been the source of tremendous fan frustration-- not just due to the fact it's still unreleased, but because next to the recently released XXX it is a masterpiece. Where XXX is sterile and vapid, Stone of Sisyphus, had balls. Where XXX is evidence of a band going through the motions making music in a vain attempt in a less than half-hearted effort to score another hit and "preserve their legacy", SoS is a band firing on all cylinders, making music they believe in for the sake of the music-- not the sake of their "legacy." And the sad thing is, if the band had put even half as much heart into XXX as they put into SoS they might still have a legacy worthy of preservation.
*three different versions of Stone of Sisyphus (in 3 different states of completion) are temporarily available for download here