You can set this up today with newsboat, if you are fine with writing a small helper script that will parse browsing links for "youtube" string and open them directly in mpv. There are a bunch of examples of these sorts of scripts on peoples githubs where they already went through the trouble of writing regex for video and image file links (beyond just youtube) for you. You then add a line in the newsboat config file to set the default browser to your helper script.

I extended one to include opening rss subscribed reddit links in rtv in my terminal window, for example.