Before Activision decided to bow out of the music game genre (to the relief of many), the publisher had one of their development studios working on Sing Hero. A karaoke game, of all things.
Pi Studios, the developer who worked on all those Rock Band Wii ports, was tasked with working on Sing Hero, up until the company closed down. Activision had actually filed another trademark for Sing Hero in 2009, but with Pi Studios' no longer existing, that pretty much went out the window.
Above, you can see concept images of female characters -- presumably playable -- from the game-that-never-was. Below are the dudes, and some very tiny screenshots. Going by the character designs alone, we're fairly glad Sing Hero never happened.
Apparently, your score in the game was measured by how many fans you could garner.