I would like the student to be able to "re-enroll" in a course; whereby, all his/her previous assessments in the course are cleared and retaken. This is good if a user fails a course that is a prerequisite to another. It would allow you to still set "Limit attempt" to 1 on any or all assessments and still allow the user the ability to buy/take the full course again for credit - and continue to the next course. Maybe this could be an optional link added to the "[course_action_links]" shortcode. (i.e check if current course is complete. If it is add link to re-enroll in course.)
This could also be added on the backend. An option to "Clear user assessments for this course" when you go to withdraw a student from a course. This would be good if you are having to withdraw the student for disciplinary action; or if the student fails a course and you would like to withdraw and re-enroll them.
Another option would be to add it to the "Course Enrollment and Cost" section. A checkbox that says "Limit enrollment attempts: Allows the student to retake this course the number of times selected."
Ideally it would be all three. I have looked in the class.shortcodes.php and class.students.php and I think that I can put together a code snippet or two if it is a feature that the developers would be interested in adding.