CoCalc Whiteboard
Just an FYI: The open-source online platform CoCalc has implemented a Computational Whiteboard that works in the browser. They have done a lot of thinking about how to do this in terms of storing the vector information, pen input, etc.
This got me thinking about being able to Draw figures in the editor as well as annotating.
Note: The CoCalc team is very responsive and would likely be very willing to chat and share ideas. (I don't yet understand exactly what they have done under the hood, but part of their design allows for keeping a history.)
The CoCalc Markdown editor looks very similar to Joplin with the added feature of being able to include executable code. It would be very cool if both evolved similarly so one could use Joplin as a local version. (I am very new to Joplin, so perhaps this is discussed elsewhere.)
Update: I just tried the js-draw plugin on my Mac, and it gets pretty close to what want. Sorry for bumping: I did not realize this was implemented a year ago!