I've been working on an iPad app in a similar direction, though with a different end goal. As I'm sure the founders of this company can relate, I've found Sibelius and Finale incredibly frustrating to use, especially for anything resembling experimentation. However, I also feel that sheet music is generally incompatible with modern popular music. (See the Aphex Twin joke below!) Much of my favorite rock and electronic music has syncopation, changing meters, pitch bending, and a general fluidity about it that's incompatible with the rigidity of staff notation. Plus, staff notation is very difficult to read.
With my project, I'm hoping to bring some of that flexibility back into the compositional process. In short, it's an infinite canvas with time on the x-axis and pitch on the y-axis that you can simply paint with your fingers. You're not constrained by key signature, time signature, or note length, though there are snapping tools to help with that if you need it. It's meant more as a tool to take musical "notes" than as something that can produce a clean final product. As someone who doesn't play the guitar very well (yet), I'm looking forward to using it to experiment with guitar solos.
I'd like to wish you the best in your project, and mention that as a huge fan of modern drum & bass and other EDM forms, plus as a longtime guitar player, I believe you're actually working in a space that lacks such a tool. The video makes me smile. That's a nifty pocket resource to me!
Just going to spit-ball a couple thoughts based on my experiences and your write-up!
I've been using Blend.io to share my Ableton Live projects with others online (via Dropbox). This is helpful because settings are exported - do you think that in time you might have a mechanism in the program to list & chart out the settings for a synth, say like Massive? Could be a whole can of worms, but important stuff to recall going forward when taking notes.
As for guitar solos, I think you could use one of the blues Kings for material in testing, either B.B. King or Freddie King, depending on how much pitch bending or note rapidity you'd be interested in. If you want to get extreme, Dimebag Darrell of Pantera had some crazy harmonic & Floyd Rose "dive-bomb" tricks he'd use that might test capability!
The initial offering will be pretty sandboxed from the rest of the production toolchain; I'm aiming for an MVP with the simple goal of "sketch music quickly". So there won't be much to offer in terms of interacting with other software.
After that, however, anything is up for grabs. Eventually, I'd love to add features that would make it an invaluable music production tool, not just a note-taking app. Under the hood, each instrumental layer in my app has a set of "traits" (volume, vibrato, expression, etc.) that get matched to the appropriate settings in the output instrument if they exist. Only the settings that exist in the output instrument are used; the rest are ignored. For the time being, these traits are only used internally for the built-in MIDI synth. But if I'm understanding you correctly, this means that it should be pretty simple to add arbitrary traits for outside instruments. And eventually, when I add Audiobus and/or MIDI-out support, it should be possible to play the track directly through the synth! (But that's a bit into the future... need to focus! Need to focus!)
Thanks for the guitar solo tips, I will definitely be using some of those!
With my project, I'm hoping to bring some of that flexibility back into the compositional process. In short, it's an infinite canvas with time on the x-axis and pitch on the y-axis that you can simply paint with your fingers. You're not constrained by key signature, time signature, or note length, though there are snapping tools to help with that if you need it. It's meant more as a tool to take musical "notes" than as something that can produce a clean final product. As someone who doesn't play the guitar very well (yet), I'm looking forward to using it to experiment with guitar solos.
Here's an early demo video: https://www.youtube.com/watch?v=Ra8OvnoxKQw
I hope to release it in the next few months, though we all know how that goes! iPad Pencil support is definitely something I want to do as well.