@pfefferle@socialhub.activitypub.rocks I believe Angus is planning on exposing tags as actors.
At the end of the day they're all going to be Actors, and your logic only really needs to validate that it has an inbox and an outbox (confirmation on that would be great), and be resolvable via WebFinger.
NodeBB exposes end users and categories as actors, currently. While tags can be followed natively, I'm not sure how easily it would be to manage that since each tag would need its own webfinger handle. e.g. what happens to a tag that is named the same as a user?