Ableton Live does this. When you group tracks, the group becomes a collapsible or expandable container for the tracks. When collapsed, all you see is the group track in the mixer, when expanded you see the individual tracks.
The folder could host the group track bits (fader, inserts, sends, etc) in the arrangement view Inspector.
It would be nice if it were a new track type and didn’t replace either groups or folders.
+1 hope