I think I would be fine with a positive enumeration. Some ideas for serving content:

    - purely random
    - sponsored but without user tracking (like old school TV ads)
       - sponsored for user selected geographical area feed
       - sponsored for user current location geographical area feed
    - follow "friends" or influencers
    - purely timeline

    - discussion boards
      - timeline (IRC like)
      - threaded
      - user votes (not magic platform votes)

    - follow keywords