The rubric grading is a very nice feature.
The same advanced grading functionalities (ex. rubrics) would be an excellent add-on to "essay" questions in quizzes.
Example of application:
I ask students to do math demonstrations on paper, scan their answer and submit the file in an essay question.
I would use this to make manual grading in a quizz with high efficiency and good feedback to students.
The possibility to comment and write on PDF is wonderful!
P.S. Not sure where to post this suggestion. Please forgive me and advise.