If anyone wants to mess around with “drawable” notation (along the lines of the “avant-garde” section), I made an iPad app called Composer’s Sketchpad[1] that lets you do just that. You can use it to sketch out complicated solos or mess around with microtonal music in a way that ordinary notation does not permit. I really need to give it a nice 2.0 makeover with full MIDI in/out, but making money on esoteric App Store software is hard!
This reminds me of Hyperscore[1], originally a project from the MIT Media Lab[2]. Had you heard of it before? If not, perhaps you could use it as a source of inspiration. I spent many enjoyable hours playing around with it before discovering other composition software such as Anvil Studio and FL Studio.
Yeah, it came up a few times! (Although I only found out about it after having already implemented the core of the app.) There are also things like fluXpad[1] which are much better suited for professional use, but at the same time far more limited in terms of expressiveness.
[1]: http://composerssketchpad.com
Bonus devblog: http://archagon.net/2016/02/05/composers-sketchpad/