e.g. I'm Joe Rogan and I release all my podcasts into a single torrent, add one each podcast. JoeRoganPodcastArchive.torrent . Each addition should propagate if the torrent client accepts it (i.e. Joe can't sneak in a episode of Oprah as the user would catch that).
Debian_amd64_ISO_Archive.torrent , each release Debian chuck it in there. Up to the user to accept the new addition (likewise download selectively, as can be done already).
By now you should get the drift of what I'm on about. The key is you don't have to get another torrent file to "follow" (what would be) the expanding torrent.
It also serves to propagate new content faster, the change comes to the user rather than the user having to seek it out.