Users should be able to create both in-play and out-of-play discussions. If the discussion is in-play, users must specify the in-play location and commenters must specify the active character they're using. In either case, the user may specify a list of participants (active characters for in-play discussions or user accounts for out-of-play discussions). If participants are specified, they are the only ones with access to read or comment on the discussion. If none are specified, it is open to the public.
It may also be a good idea to tie this into the old "Follow" feature, automatically causing users to follow and receive email updates on any post where they're specified as a participant.