Provide a way to display and to change the start point or flow direction of ductwork or piping so branch lines connect correctly. Adding this tool to the contextual ribbon of the selected item(s) may be a good place to put it. Also displaying an arrow in the direction of flow will help as well. This to could have the flip toggle associated with it.
Sometimes you don't always model your ducts or pipes in the correct flow orientation so when Revit attaches the branches it's fitting is going in the wrong direction. I know it's easy to flip the graphics of the fittings in Revit but if this is not correct when you go to push to Fabrication the fittings orientation is still not correct and just adds extra time to fix.