Keep Circulating the Tapes: Their unreleased 1973 album A Kick in the Head Is Worth Eight in the Pants, is a highly sought after collector's item. It was recorded at the group's lowest ebb of popularity, and the record company, smelling another flop, refused to release it. It still hasn't been given a proper release to this day, which is quite a shame because musically, it's a very strong album, albeit rather on the somber side.
If you just want the music, their 1981 album Living Eyes is easy to obtain, as it's available for download from a variety of sources. However, if you want an original CD copy of the album... good luck. It was one of the first compact discs manufactured and released; however, the format was in its infancy, and by the time the CD became the dominant music format, the album had long fallen out of print. To date it has never been reissued, and is extremely rare.
What Could Have Been: In the late 80s, there was talk of youngest Gibb brother Andy Gibb joining the Bee Gees, making them a quartet for the first time since their late 60s harmonic rock period. Andy's untimely death at the age of 30 in 1988 meant this would never come to pass.