Extension panels in the Stream Manager
I want to be able to make a custom panel that a creator can put in their Stream Manager.
It already looks pretty extensible with how you can move stuff around and pop things out.

-
nixxquality commented
The request is specifically for your latter case, inset panels in the dashboard.
Here's a quick mockup I threw together with developer tools to show what I mean: https://i.imgur.com/zGg0Glp.jpg
My idea for the extension is something only the broadcaster would interact with.
-
You can make a panel that only the creator can see from the Creator Dashboard, but these are popout windows for active Extensions. Does that satisfy the use case, or is the request to have them be inset panels in the dashboard?
If the former is fine, then this is possible using the "Live Configuration Path" from the status section when editing a specific Extension version. Only the broadcaster sees this view and it can be opened from within the Quick Actions panel.