Add more channels to more instruments
Now that the Mixer so nicely collapses multiple channels into a single column (that can be optionally expanded by clicking the button at top left of the column), maybe it's time to consider actually adding more channels to more instruments? It comes up often enough that someone wants a bunch of channels for the different sounds in an electric guitar, say, or mute (con sordino) channels for brass other than trumpet, etc. We hesitated in the past for fear of cluttering the mixer. Now that this isn't a concern, maybe we go for it?
Going a little further...
While we could go in and brute force add channels to each entry in instruments.xml, this invites all sorts of debate as to which channels to add for which instruments. What if instead, we added a "New Channel" button to the mixer? Yes, we already have the ability to add channels via instrument change text, but these channels are not available for use by staff text, which greatly limits their usefulness. Of course, I wouldn't mind seeing that changed either, but still, adding channels from the mixer seems natural to me.
Failing that, what if we just automatically added a couple of extra channels "User 1" and "User 2" to each and every instrument when adding them to your score? That is, if instruments.xml defines one "normal" channel, we give you "normal", "user 1", and "user 2"? If instruments.xml define normal/pizzicato/tremolo, we give you those plus "user 1" and "user 2", etc. If you never open up those columns in the mixer, you'd never need to know or care they were there, and I'll bet this would be easier to implement than a dynamic facility.