1. Make the color/clear buttons a little larger. The hit targets are very small at the moment when using a Pencil on iPad. It's easy to draw on the buttons by mistake when you mean to press them.
2. Add a “stop annotating screen” button to the buttons, separated by some space so it's harder to press accidentally. That way I can end the annotation session from the iPad instead of having to use the keyboard or mouse.
3. Add a “undo stroke” command and shortcut. Users of AstroPad should then be able to map this to a two-finger tap, which would give a really nice undo workflow for drawing on screen. https://astropad.com/magic-gestures/. When annotating it's pretty common to want to undo a bad mark without having to clear everything you've drawn so far.