A few from me.. all IMHO only
Represent bars properly when dealing with unusual time signatures - one bar per bar in the chord sheet window
Either.. drop the instrument names in trhe GUI just call them instrument one, two, etc or, give them the names that actually apply.. don't call banjos pianos
Assign most of the MIDI readings to a loadable window, most MIDI assignments/program changes are rarely used, weho uses LSB?
Build the GUI for Real Tracks its the future...
Use more tooltips - color coding things, though useful, is only good if you have read the manual.. did you read the manual for yur mobile
Get away from the windows 95 look it creates a poor impression.
Make it possible to show repeats in two ways - one as in classical notation, secondly in exanded view as if in a sequenccer
Last but certainly not least, a great learning tool would be to provide a learner with explanation of given chord types. E.G. Bb7Alt is exactly.....? Do this directly on the chord sheet window by a hovering action (which may be turned off in prefs. Provide some suggested chord voicings somehow - for learners. Also consider indentifying chord sequences as groups - e.g 2/5/1s chord subs, modulations. In other words try to display the structure beyond roman numerals (which are great).
Loose the windows 95 look its bad for your image - a great engine in a model T chassis
Love the product would love to see the GUI brought up to date