Whenever I want to make a fill on a new layer, I am forced to go to the layer that provides the colors I want to sample from, make a selection there, then go back to my empty layer and then color the selection. This is kind of a lot of work for a fairly basic functionality that is crucial for coloring line art, where you have to create lots of separate color layers.
I know painting is not your priority, but from another point of view, seeing how you like to advertise this non-destructive workflow (which I LOVE btw!), I do not see much use for a tool that is only able to realize its full potential (namely edge detection with tolerance) when it is allowed to overwrite your base layer.