Is there any public info about the “tags as genres” work going on?
Tags as entities makes sense to me. Every permanent object should be an entity… Hell, as one of the original developers of the Drupal Relation module, we even made relationships entities, so that you could have relationships between relationships(!).
Yes, I tagged CallerNo6, because they mentioned the idea of a vote-based similarity metric. e.g. anyone can add a tag-tag relationship, but only when there are enough of a given relationship is it considered a synonym.
It might also be worth looking and StackExchange’s tag synonym process.