Hi.
This particular thread IS about the look of the sidebar panel though.
There would be no point of redesigning the “sidebar panel” without fixing this annoyance: the “tab clutter”.
IMO, the right location for add-ons is where they currently are: in the N Panel.
I don’t see how, allowing add-on devs to place their add-ons in the Properties Editor would fix the clutter. It would just move the clutter in the Properties Editor.
The N Panel is a better suited area because add-ons are very accessible there, being very close to the center of the viewport: the distance between the scene and the N Panel isn’t that far, which is good for a speed workflow.