Hi I am an indie app publisher/developer and have a music theory exploration app on the App Store, but that's not why I'm posting here today. There are already tons of apps for learning music, playing music, interfacing to MIDI, etc.
What I am interested in is what are the most crucial features you would expect from an app for exploring music theory? Wish lists of features for existing apps, or features from apps that already do this well. I am not interested so much in names of apps as much as the features that people would really like to see. I would like to compile this list to share with all app developers so that we can take this genre to the next level.