Hi!
I’m more then grateful for the introduction of the Rating element with schema.org support.
Just deleted a ton of code in favor of the new element 
After playing around with using different settings and validating them against the schema.org specifications, I would like to suggest to add some attributes.
Review Aspect - allow sub-ratings
If ‘reviewAspect’ would no longer be the now hardcoded ‘General Review’, but adjustable through a text-input field, the door would be openend to use the Rating Element for sub-ratings as well.
Single and Aggregated ratings - they go hand-in-hand
A little more complex would be distinguishing single and aggregated ratings. I do believe this could be worth looking into; when the Rating Elements is used to display the rating for a single review, chances are a calculated aggregated review score should be displayed as well. To accomplish this, we would need to add a toggle button between the ‘Item image’ and the ‘Author Name field’. This could a field with label ‘Rating Type’ and values ‘Single’ and ‘Aggregate’. The ‘Author’ fields should then only be shown when the toggle is set to ‘Single’. This would require a bit more work then adding the previously mentioned ‘reviewAspect’, but I do believe it would make the Rating element even more powerful.
Full compliance with schema.org for all possible Types
There’s one more suggestion I would like to post. Using different values for ‘Type’ results in different (not yet existing) schema.org attributes to be required and/or recommended. It would be impossible to develop and maintain the Rating Element in a way that it provides settings field for all possible required fields. I do however imagine a solution might be possible, bringing more flexibility into the element. Something like a checkbox that says ‘Additional Attributes’. When checked, a table with two text-inputs on each row enables the user to define additional attributes and the value for that attribute, through text-inputs.
I do believe that by enabling users to add any schema.org attributes they would like, the Rating Element could be implement in almost every user case.