Coloring of unplayable nodes
I would like that when defining a new instrument in the xml-file, then it should possible define what notes that the instrument that can be played. In most cases, this is already possible: The user can define the prof-range and amateur-range and nodes outside these ranges will be colored red/yellow. This is not always flexible enough. A natural horn forexample can only play 5 notes (for amateurs) and a few more for profs. These playable notes are not an interval.
What it will add is that it then would be easier to spot mistakes, as the unplayable notes will be colored red.
Comments
There are other enhancements that would be useful too, like indicating notes that are not hard to play, but as only possible if you have particular models of the instrument (like low A on a baritone saxophone).
Meanwhile, probably it would be easy to develop a plugin for specialized cases like these.