Suggestion (possibly for plugin creators): Split [Instrument] into [Instrument I] and [Instrument II] and vice versa
As a composer/arranger who mostly works with choral music, it's sometimes very useful for me to split and merge the vocal parts. For example, the Soprano part might need to split into Soprano I and Soprano II for a few measures and then merge back together.
As best I can tell, the only way to do this in MuseScore at the moment is to have three parts on the score marked as Soprano, Soprano I, and Soprano II, and then make extremely judicious use of the "Hide Empty Staves" tool. This makes the engraving process for complex choral music agonisingly slow.
This is definitely something that I would use very frequently in my work. It might not be top priority for all users, since I believe this kind of part splitting is very uncommon outside of choral music, but it would be an enormous timesaver for me.