I work a lot with PDF-files (most DIN-A4 / Textbooks etc.)
It would be great to have a horizontal expansion option (if you´re on vertical scroll)
For an A4 vertical baseplate one could do a vertical expansion with a DIN-A page of another size. (also would not have to be DIN Normed could just be any page)

It would be great for taking notes or adding additional information in textbooks.

If one would work through a text book it would be easy to just select all the expansion pages and copy them into a separate Notebook. Particularly for summaries.

The new Notebook could also have links to the original Page and expansion for the Textbook.

When working with work sheets you could add a smaller page and wouldnt need to scroll up and down /switch tabs /open another window all the time.

I also use the lasso tool with copy fairly often as I dont want to make stickers all the time for smaller projects. A small expansion sheet on the side would be a life altering experience to be honest as some sketch books of mine have whole Pages with just small notes or copy sketches.