Something like this would be great if I could add an element into a digram that would allow me to explain the business context for a given set of interactions in the diagram.

For example, perhaps a given API is only called conditionally. Being able to note what that condition is would help me communicate the business logic.

Functionally this could be done by leveraging the current description field on most entities. But the problem with the description field is that it is buried at the bottom of the very busy editor panel and C-suite folks would never use that. If descriptions were accessible directly on the diagram that would help.

Perhaps the items on the diagram that have a description could be highlighted with a special visual treatment like an asterisk or little icon. Then if we could have a pop-up in diagram that shows the description text when you hover over that indicator we could train the exec teams, product managers, etc. to use that as a way of knowing there is additional context in the description and you can see it by just rolling over the indicator.

Also - the ability for comments / notes like this to be visible to read-only viewers (it currently seems only editors can see them?)

I've tended to use the diagram and/or object detailed descriptions for this but you have to know to look for them there.

Could even be an extension of the comments feature, just a different category

Some kind of "free text box" or "sticky note" that we can link to an object would be great :)