We use submaps heavily, and we would want that "Map navigation tree" widget would dynamically reflect changes on our maps, i.e. when we add new "Map" elements on existing maps.
I think it could be possible by adding a "dynamic" checkbox when creating "tree element" on the widget. Then this "tree element" should not have child added manually, but render child only automatically.
Now we have to manually edit the widget each time when we add/modify sub-maps, which is not very nice.