I would like to be able to add a child field by right-clicking a parent field in the "all fields" section of the properties pane.

 

You could add an "Add child field" option to the right click menu.

 

When "Add child field" is selected, the Field Properties dialog box would come up, and the "Parent ID" field would already be filled in with the ID of the Parent field that I right clicked earlier.  Then I would just fill in the name of the new child field, choose appropriate options, etc.  Voila, new child field.

 

Would also be nice if the Field Properties dialog displayed the Parent Field Name alongside the Parent ID number.  Would certainly be more informative.

 

Also in the All Fields section I'd love to be able to arrange the fields in a hierarchy using the same keyboard shortcuts that we use to arrange items in a grid.