a sugestion for a feature that would be very simple to implement, but will help the user in the creative process significantly.
In piano-roll the piano-keys should be right-clicable. This should light up a bar, in the length of the piano-roll, and the bar should be kept lit up until the same key was right-clicked again (and/or a button/option in 'Edit': 'remove-all-key-marks' was klicked).
The user clould this way mark the notes that he finds useable in his composistion.
This feature is a variation of Reasons 'collapse-keybord' where only selected keys are visible in piano-roll. Sleaker, but that would also be more difficult to implement