You'll be able to add elements in the corners with visual templates
I'm afraid that the current Diagram capabilities are not a good match for the rest of your requirements.
You won't be able to display the shape itself in a tooltip and to make the children collapsible.
A hover event is also missing, but it can be substituted with the select event
for the time being.
Please, file a request for this features on our UserVoice portal
if you have minute.
This will help us gauge the overall interest and prioritize.
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