Add soloing and muting presets and management to the mixer
Often one finds oneself soloing (or muting) the same groups of instruments once and again. For instance, when composing (or teaching, or studying...) an orchestral work we need frequently to listen to the strings alone, or the woodwinds alone, or the combination of two soloists, or anything like that). The mixer could have two buttons: one to save the current solo/mute settings and the other to load a saved setting. It could provide some standard options as well, such as solo strings, solo widwinds, solo brass, solo percussion. This feature would save a lot of time.
Comments
Refer to the discussion about mixer redesign here: https://musescore.org/en/node/289790
Where one of the proposed changes is to include grouping instruments by category/brackets. Allowing Solo/Mute on those groups would already get very close to what you're looking for.
FWIW, one way I do things like this currently is to create "parts" for the different sections.