A feature I would like to suggest is being able to add comments/notes to individual annotations within an assignment. It can get quite confusing and convoluted when you have lots of annotations, highlights, underlines, strikethroughs and then for each of these you are also adding a 'note' which can often cover up some words on the assignment. If you could just add a comment on the annotation (much like on a Google Doc) that would be helpful.
I would also appreciate being able to light up marking criteria/rubric criteria to certain segments of students' work. That way they can know exactly WHY they have achieved a certain grade.