Hm, I think this is a bit of a big hill to climb. Jack-of-all-trades programs often run into issues of being less specialized in every area than other readily available tools, rendering them less effective to use than approaches that combine multiple platforms.
Music creation in particular is really quite complex to tackle in a standalone. Having the ability to make midis and midi instruments from imported sound clips (either by autopitching or by letting the user import a new clip for each note) would be really good though. In fact, it would put you into territory very few have effectively covered... but making intuitive controls would be a real challenge.
For an example of how not to do a jack of all type program, look at novelty the vn creator - sure, some people really like it, but the UI and explanations of how things work are *horrible*. Successfully making a new character for the first time practically needs a PHD.