Allow page-level application extensions
Currently, application extensions can work with known locations, and with toolbars for lists, but there is no (supported) way to create extensions that would allow you to add toolbar buttons to pages.
If we want to add a button that is visible when a user is editing a modern page so that they can do stuff within the page, there isn't a (supported) way to do that.
For example, I want to add a button for my content editors that verifies their content for WCAG compliance, or make sure the content adheres to corporate guidelines before they save the page.
Extensions are awesome, and we love SPFx. This would make things even better!
Hugo Bernier commented