This idea has been developed and deployed to Canvas
As an Instructor, I would like to allow students to resume a quiz where they left off after it as been submitted when giving them another attempt. How it currently works is that they need to be given another attempt and start over. If an instructor could choose to let them resume their previous submission from the moderate page, it would save time for the student on long quizzes. Also, it would be nice to have an option to lock previous answers; that way they cannot correct previous mistakes.
As a student, I would like to be able to resume a quiz where I left off if my instructor permits. If I got most the way done with a 100 question quiz then have some technical difficulties, it would be a time save to not have to re-take the entire exam.