This sounds like something I was suggesting for Amethyst. Visually, I don't know how to display it necessarily, but I do think there is a lot of value in independent user tagging. This would make hell threads more than a list of users. As well, it would clean up a lot of stray kind-1 events which only exist to "include" a user in the discussion, or otherwise bring their attention to it github.com/vitorpamplona/amethyst/issues/1259