Maybe what you want to do is to make it so that people could create their own discourse (e.g., username.yoursite.com) that runs on a multisite installation that has your “home” discourse as the SSO server for the user sub-site. You could conceivably create a plugin that would create a new username.yoursite.com discourse site whenever a new user got created. But maybe every user doesn’t want their own sub-forum and you’d instead have . . . something . . . that would let them create their sub-site.
I think that’s starting to get close to what you’re trying to do.