The ability to edit system families would be massive.
For example: The Cable Tray System Family has an incomplete Wire Tray (just a solid U shape for all pieces). Revit will allow you to edit all aspects of this (T's, Cross, Bends, Reducers, etc) via a dropdown box where you can select which tray you wish to populate for that particular piece. It gives you these options for all the connections except the straight run (which leads to it just populating straight runs with the Standard Wire Tray found within revit which doesnt represent a wire tray at all). Adding a drop down box for the straight runs in the system family would make it fully customizable and should be an easy addition to the existing software. This would allow companies like myself to use our own desired families to create easy systems.