In an interview with Ars Technica Harmonix' Alex Rigopoulos revealed that the studio had been talking to rights holder Sony about a follow up to their PS2 title Amplitude (a sequel of sorts to Frequency).
"Ars Technica: Has there been a conversation about bringing that game to the PlayStation Network? You've proven the formula, and you can say "From the people who brought you Rock Band."
Rigopulos: Yeah! Yeah! Yeah!
Ars Technica: It seems like it would be a mutually beneficial thing to do.
Rigopulos: I would love to. That game is still close to my heart. I love it. I'd love to do a sequel to Amplitude, actually. It's an issue of prioritization. It's challenging enough to keep the Rock Band train on the tracks. (Editors note: earlier that day, Rigopulos had announced that there would be no new Rock Band sequel in 2009, in order for Harmonix to focus fully on the Beatles title.) We've grown from about 80 or 90 people a couple years ago to more than 300 now, and managing that kind of growth takes focus.
For us, that lingering question of "what are we going to do about Amplitude" is still very much out there. I would love to come back and do it right for the PlayStation 3, for example. "
Source: Ars Technica: Harmonix interview