Harmonix may be keeping itself busy with Rock Band Blitz and a mysterious next-gen game, but at one point the studio was moving into more band-focused specialty games. Where do you go after The Beatles? The same place rock and roll did, of course: Led Zeppelin.
A resume from Superannuation mentions "Harmonix Rockband Led Zeppelin" among its credits under trailers and commercials. The resume comes from a former employee of Passion Pictures, which worked on the cinematic sequences for Beatles Rock Band.
It all seems to indicate that Harmonix was once working on a Led Zeppelin game, but that's been dropped. The resume doesn't mention active dates of the project, but it seems likely it came sometime after Beatles Rock Band, but before Viacom sold the developer back to itself. The music game market may not have been strong enough to support yet another big-budget game like Led Zeppelin, so we're unlikely to ever see the project.
Update: Harmonix later offered Shacknews this statement: "As we've noted before, we're constantly in talks with all major artists and labels. That said, Harmonix has never been in development on a Led Zeppelin game, though we'd obviously be thrilled to have them in Rock Band."