Somehow, I would love to have a different icon (or even a different color?) to easily distinguish between local component instances & instances from external library components.

This would help when joining a team or project or when revisiting a file after some time.

Looking at the layer tree right now, you have no chance to know if a selected component instance is local or from a remote library. Only in the right-hand Design pane you can see if you can click through to the component master in the same file or if it will open another file.

My first idea would be to simply introduce a new icon that plays well with the existing master/instance icons

Building on this, a separate color for remote components could be even more beneficial, since you could then spot remote components just by the color of the selection in the canvas.

What do you think?