The feature I am suggesting is a new annotation tool similar to the loupe feature of the Preview application on MacOS devices.
This tool allows the user to create a circular region on a document being edited that enlarges ('zooms') the document elements underneath it, to show detail.
For items that are underneath which are fonts, SVG, or other scalable formats, the zoom appropriately scales those elements upon creation or release of the tool. For items that are underneath which are images with a higher source resolution than what the document is showing, the zoom appropriately re-renders the zoomed region under the highest resolution available for that level of zoom.

This circular region can be moved by first tapping it to show the edit-view of the region, and then dragged to a new area. The region can be resized with a small circular button on the boundary of the region by dragging this button inwards or outwards to change resolution.
The region's zoom level can also be changed with a different small circular button on the boundary of the region, by dragging the button clockwise along the boundary (for zoom level increase) or counterclockwise along the boundary (for zoom level decrease). These buttons only appear once you enter edit-view of the region, by tapping the region once.

A suggested enhancement to this feature would be to allow the region to be (optionally!) rectangular rather than circular, if the user selects this configuration. The MacOS or iPadOS versions of this feature do not allow the region's shape to be changed from a circle. Perhaps GoodNotes could implement multiple adjustable shapes.

THE MOST IMPORTANT PART OF THIS FEATURE is that these zoom regions PERSIST when you save and close a document, export it, or share it. That means this zoom feature would be more than just a document viewing tool, like the current GoodNotes 5 zoom window, but an annotation (editing) tool. The changes would be saved to the PDF document or image document.

This can be achieved by rendering the circular region once and saving this region as an image so that it can be embedded into the document or imposed on top of the image file.
Locally, in GoodNotes, it would be even cooler if GoodNotes instead saved the region as part of the notes file, and only flattened (rendered the circular region as an image) when exporting the file or sending it somewhere. This way, the region can be moved later if reopened in GoodNotes.

Please see the attached screen recording of the iPadOS "Loupe" feature being used on an image document. I am optimistic that the GoodNotes team can come up with something greater that fits the use case of GoodNotes and meets the need of its users.

Thanks!