Currently, MyPaint has an overview/navigator panel which lets you view the whole area that has been drawn, see where you’re currently drawing (ViewFinder), and move around that area.
I think this panel could have a true “preview” mode in addition to the navigation mode. The differences would be as follows:
Displays overview of the whole drawing.
Clicking somewhere moves the view to that place.
Settings: display/hide viewfinder.
Lets you set the width, height, OR dpi of the preview. The drawing (clipped by the frame tool boundaries) is resized to match. Anything drawn beyond the frame boundaries doesn’t show in the preview.
Clicking somewhere lets you drags the viewport of the preview around if the preview is larger than the UI panel. There’s a checkbox to make the preview viewport follow the main viewport automatically.
Example of usage: you’re drawing at 600dpi, but you intend to export the image for web at 90dpi. The preview panel could show the actual preview of the drawing, allowing you to draw in detail while seeing what it would look like when exported.