As a musician, I quite liked the GH Tunes/GH Mix thing they introduced in World Tour. Sure, it was pretty much a MIDI composer in many ways, but it had a lot of potential and was quite fun to use once you got the hang of it. I think the guys behind it actually did a really good job considering what they were working with.
My own creations:
If they focused on that side of things for a small title, there might be something there; because that's certainly not an area that Harmonix are covering.