What problem does this address?

One feature that is available in the classic WordPress Customizer experience for many themes is the option to “Automatically use the first image in post” as a fallback for featured images. Bringing this option into the new Post Featured Image block would also shorten the product gap in the editing experience with block themes today.

What is your proposed solution?

To expose a setting in the UI (Block settings sidebar) to allow users to select this option as a fallback when no featured image is selected in the post.