Like in Apples Keynote App, when connected to a beamer I have the posibiltiy to make annotations for the current slice. There are different colored pens and a kind of Laserpointer tool (a glowing red dot or so). This is great for demonstrating things, whithout manipulating the current slice.

Same would be great for GoodNotes - as a teacher I use GoodNotes as a Replacement for the traditional board. At the moment I do a workaround by undoing the pencil strokes I made for showing things on the current sheet.