I'm wondering if there is an evolution/extension of hashtags to be had. Hashtags are currently opt out from appearing in feeds, and opt-in to subscribe to all posts, added column, or searched for it etc.

I'm wondering if there is space / need for one which is only shown if a user has opted-in.

For example, #popularMoment and then %popularMoment (or w/e symbol) would be unlisted and only show up as described above.