While we're thinking through block-based themes and their templates, It might be worth expanding the concept of what a "Featured Image" is.

In the past, the featured image was a single image, added and managed separately from the rest of the post content. For block-based sites however, that restriction is largely unnecessary. Rather than just an image, we should consider making it possible to set a video or audio embed as the "Featured" block. Also, since posts are made up of structured data, it's theoretically possible to signify any block from within post content as a "Featured" block.

A couple possible use cases:

• A photo blog, where posts contain a single image. If no separate featured image exists, the first image in the post is used as the featured image.
• A podcast site, where each post's "featured" block is audio of the podcast episode.