Shirley Manson of Garbage was supposed to do a solo album but her label didn't like the material she brought forward, so the project was scrapped. I think they wanted pop hits and that wasn't what she had in mind. Still kinda gutted she didn't get her way.
The third Spice Girls album was supposed to feature solos and duets in addition to the types of group numbers that had defined its predecessors; I think they'd wanted to slowly introduce the world to themselves as solo artists. But then Geri left the group and we got Forever a few years later instead.
I remember reading something somewhere about Madonna working on a rock album after American Life. Not sure if there's any truth to that, but I bet it would have been really cool.
Scissor Sisters pretty much *completely* scrapped what was meant to be the followup to Ta-Dah and recorded and released Night Work in its place. Apparently they hated the original followup so much that it almost caused them to disband.
