Custom data attributes (data-custom-*) per element
Support for data-* attributes with custom names would be great.
It would allow keeping 3rd party application specific metadata along elements. This would enhance sync possibilites with external systems.
These attributes would be not interpreted by OneNote but persisted. Now I tested it and these attributes are thrown away when doing roundtrip via OneNote API.
Currently, there is only data-id attribute which is great but not offering much flexibility.
These new attribute could have extra prefix e.g. data-custom-* not to collide with your internal attributes..