I would dearly love if someone could/would make a plugin for YouTube-DL. It's open source, written in Python, does NOT break DRM. I'm not a coder, but since it's pretty much self-contained already, wouldn't that mean all that needs to be done is interface it with Calibre? I admit I don't know; it has a huge list of options I don't have a clue how to use. If it could play nice with FanFicFare on AO3 fanvid entries, that would be even better.
http://yt-dl.org/
https://github.com/ytdl-org/youtube-...ing-youtube-dl
https://github.com/ytdl-org/youtube-...ovies-for-free