MuseScore is great piece of software. Having sheet music creator of such capabilities is very very nice thing. It would be nice if it could become accessible for blind and visually impaired people. Yes, there are several ways to typeset music using text input files, but an interactive tool has some advantages batch tools can't provide.
So here are things I consider important:
1. Full operability via keyboard - object navigation, selection, creating/editing objects accessing objectproperties - all that without need of mouse. That does not mean I hate mouse - I use it often.
2. Support for screen readers - Orca on Linux, NVDA on Windows, and fully documented interface for other screen reader vendors.
3. Insertion point should be always visible regardless of zoom level and it should be tracked.
4. MIDI support on Windows including real-time recording. That is not directly connected to accessibility, but helps a lot in general.
5. System theme obeyance - it should be possible to use system colors for notation objects.
I'm looking forward to see really great tool for musicians.