Add Instruments to existing parts
As discussed here: https://musescore.org/en/comment/914161#comment-914161 it was nice having the possibility to add further instruments to existing parts - also after the moment of creating them. While doing arrangement work I like working with "subscores", e.g. a part containing the whole sax section and additionally the piano. If I want to also include the bass later on, I had to redo the whole part. Really uncomfortable and a bit annoying.
If that was too hard to do, maybe the following suggestion would do it, too:
The possibility to create a new part based on an existing part ("duplicate part and edit"). So the existing part would not need to be touched, just the list of included instruments would be extracted and copied to the new part's list of the instrumentsInPart (before actually creating it)