I think it could maybe be an optional inspector() function in registerBlock that leverages Fill behind the scenes. I think we should not be prescriptive about what you can render in the sidebar, but provide basic form components eventually.
Examples for this use case:
Somewhat related to #609.
Here are mockups of how the inspector could work.
When no block is selected, you see the normal sidebar:

As soon as you select a block, the sidebar shows the inspector:

To get post settings back, just click anywhere to deselect a block. Or you can click the "Post" breadcrumb in the sidebar. Clicking the breadcrumb would also deselect any selected blocks.
Also possibly related, #352
See also #952
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Here are mockups of how the inspector could work.
When no block is selected, you see the normal sidebar:
As soon as you select a block, the sidebar shows the inspector:
To get post settings back, just click anywhere to deselect a block. Or you can click the "Post" breadcrumb in the sidebar. Clicking the breadcrumb would also deselect any selected blocks.