For use in a music education settings, also for music editors, annotations would an extremely useful feature. That is, the ability to add markings that are visually distinct from the score itself and can perhaps be enabled/disabled in one stroke. To some extent, the visually distinct aspect is doable by using text styles - colored text, possibly a frame with colored background. And the enable/disable aspect would be doable with layers if this feature were truly supported (and worked with a broader range of markings). But we would also need a basic set of drawing tools - circle a note, for instance, or draw a box around a given measure, etc.
I propose adding a new Annotation element and some sort of UI for basic shape drawing - maybe just line, circle / oval, and rectangle to start with. Unless we want to truly support the Layers mechanism - and make it easy to use! I would also propose a menu item to enable/disable display of them. Unlike View / Show Invisible et all, this would affect print, not just onscreen display.
If we can find the right balance of simplicity and utility, hoepfully it would not be a very large implementation effort. It is something I would be interested in trying to make happen for 2.1. Thoughts?