I'd love the option to save the thumbnail URL instead of saving the file on my server. A very simple documentation would suffice and I could patch other plugins to retrieve the video thumbnails from their original servers when needed.
The current retrieval of the thumbnails might even be problematic in regard to copyright, but at the least it uses bloggers' servers resources needlessly. For example Top 10 supports showing video thumbnails after this patch. When the Top 10 thumbnail is loaded, the whole image is transferred using the blogger's bandwidth. When the Top 10 link is clicked, the embedded video loads the same file a second time from the video host, eg. Youtube.
Instead of this it could be just hosted in Youtube and loaded only from there and only once. This would not only also avoid legal issues, it would save disk space as well, making backups easier etc. Not a bad option to have in Video Thumbnails. Should also be easy to make: just save the thumbnail URL to a database field and provide users the information what function to use to return that URL.