We already have sketching apps for writing, painting, drawing, and even...sketching. There are plenty of fun, entry-level apps for people that just want to experiment with musical sounds. What about a simple app that does basic music notation?
I think a full-featured, MIDI-capable, multitracking app with full playback capabilities would be overkill, not to mention unprofitable. But something that would work as blank score paper, with maybe up to four staves, output to MIDI so you could take the file off the iPad and put it into a more robust tool like Finale or Sibelius or similar...there are possibilities there.
I've taken a quick look at Symphony, but I'm not interested in an iPhone app for the iPad. Might fill the bill if it can do what they say it'll do.