In one job listing, Harmonix is looking for "an accomplished Narrative Designer to incorporate creative story lines into gameplay on a unique motion-gaming IP." The developer is also looking for content and level designers, as well as a software engineer.
What could the game be? It sounds like it could be something like Space Channel 5, except with Dance Central-styled controls.
Harmonix recently Rock Band Blitz, a variation of Rock Band that doesn't require plastic instruments.