I would love to see this feature in the track editor:

When part of a track is selected (marked yellow), selecting an instrument, effect or volume level (clicking the icon with the mouse) will apply that change to all selected notes.

This should work only if the instrument/effect/volume column is in the selection.
Otherwise, selecting a instrument/effect/volume will only change the current volume/instrument/effect.

E.g. when the selection is like this:

The user can change the volume/instrument/effect for the selection by pressing the icons.

But when this is the selection:

The user cannot change the volume/instrument/effect for the selection by pressing the icons, because
those columns are not selected. Instead, the current instrument/volume/effect is set.

PS: technically it probably not difficult to do the same for the octave column, but I'm not sure that's a very useful feature. There's already a better way to transpose notes.