Allow invites and spectators
This is similar to the "allow followers" idea already entered, but would allow a GM to send an invite to a user to participate as a spectator, or (I guess) a "follower", able to read all through a game and possibly post comments (if given that permission by the GM), but not to post moves or the like.

Thanks for the suggestion! The plan is to soon allow you to open up your game to outside viewers, so invites won’t be necessary. It will instead be a switch that the narrator can throw to either keep the game private to its participants or open to anyone who has the URL.
-
Rachel Wild commented
it's good to know that this functionality is planned Stephen, thanks, there's quite a lot of us using workarounds to achieve the same thing at the moment (see for example: https://storium.uservoice.com/forums/211322-general/suggestions/6229926-link-to-retired-characters-and-or-games-either-on )