What problem does this address?

If the user adds headings in the wrong order (for example starts with an h3 followed directly by an h6), there is no feedback about the wrong structure.

What is your proposed solution?

Users should be warned about skipped heading levels.
This could be done as a notification, or if they have a table-of-contents block it can happen in there as well. Since the TOC block will be parsing the headings, it will be possible to detect any abnormalities and inform the user so they can improve their structure.