In April this year the company stopped releasing weekly batches of DLC for the Rock Band series after 275 consecutive weeks of content. "In recent months we’ve scaled production down as we’ve transitioned resources onto other projects," said Harmonix at the time.
Rock Band 3 was pretty solid -- so much so that a sequel isn't really necessary, unless it was a new band-specific game, like if they'd gotten the rights to Led Zeppelin, for example. Considering this is something brand new, well, I have my doubts. You already had me at the band stuff... don't much care for anything else related to music that would make a viable gaming franchise.
@BryanParksSuper You know nothing. Harmonix have been making rhythm games since 2001(even before that if you want to count their failures before Frequency), WAY before popularity ever came to the genre. That and the fact that most if not all of their games are of the rhythm genre suggests that this new game has something to do with rhythm. I don't claim to know what they're releasing, but given their history I would think it's very likely.
Given a combat designer, it could be something as simple as timed hits to the rhythm of background music or something. Who knows with them. They could just try something non-music related altogether.
@fredyellowone @BryanParksSuper I personally still play. I plug an AG Riffmaster to my PC and play Phase Shift with it, which is essentially the same thing as Rock Band. I make my own song charts too so I get virtually infinite replay value out of the whole thing. No filler songs to skip and everything other than the controller itself is free.