Harmonix are best known for their work within the music genre; in fact, they're best known for making the music genre what it was. They worked on the early entries in the Guitar Hero series, they hand a hand in Karaoke Revolution and, more recently, proved that DLC could be awesome with new, weekly songs for the Rock Band series.
So it's no surprise that they'll be sticking with the music genre next-gen, it's done them well so far. What IS surprising is that they're hiring people to write an engaging plot, somebody to design levels and scripted events and somebody to work on combat encounters for a "next-gen music-based motion-gaming IP."
Being a big fan of Rock Band, I can't help but be a little bit apprehensive of what they're going to announce, Guitar Hero: Warriors of Rock was enough to put anybody off a story-based music game. Still, this is Harmonix we're talking about and, generally, they manage to hit all the right notes.
Source: Save and Quit