I’d love to see an optional comment/annotation field on containers, where the comment becomes visible directly in Structure Mode.
Why? Because on complex pages I sometimes feel like I’m opening mystery boxes! click, click, click, just to figure out what that specific container is doing.
A small visible note would instantly show the purpose of the container you’re looking at, saving time and sanity. It would make pages much easier (and more fun) to understand at a glance.