Having a ruler like in AppleNotes would be really helpful.

Besides that, as a student with a lot of maths, something like GeoGebra in a simpler version would help too. Being able to enter a function term and get the function in a coordinate system would be of great help and an even greater addition to GoodNotes.

Also, being able to select multiple pictures to insert would be helpful, as it would save time from having to select each and every single picture.

In PowerPoint, the function of print as handnotes gives the user three pictures on the left hand side and space to write on the right hand side. I think it would help to be able to insert PDF's, PPT's etc that way too, and not only having each slide from the file as a slide in GoodNotes.