It would be nice to have automatic thumbnails being generated automatically, kind of like how the Patterns in the Block Editor work. Currently, the user would need to get a screenshot of the template and upload it as a featured image, a process that is not ideal.
I would like to see this as well. Not sure if it’s ideal loading wise.
Integrating GenerateBlocks with API Flash would be a way to handle the automated thumbnails. The free API Flash account (100/mo) would likely provide enough screenshots for most websites. https://apiflash.com/