I’m still punting for a user list on a directory level. 
https://DiscourseDiscovery.org -
Anonymous users will see a directory of forums who have registered to be included in the public list (searchable by category or ‘Forum tags’), and also Search keywords across all instances in the directory.
Sign up to create a DiscourseDiscovery user account. New accounts will start at Hub_Level_0, and can progress through the remaining Hub_Levels by completing certain actions and adapted Discobot ‘Forum Training’ covering all the necessary skills you think people will need.
Different Hub_Levels will allow access to more features across the directory. Eg. Seeing categories not normally visible to anon users, take actions such as Like, or vote in polls (avatar images could be circled with red to denote associate/Hub_Level members), or get notifications on topic content, etc. Tapping on an avatar would bring up their DiscourseDiscovery Usercard.
Users can then add any ‘full-member’ forum accounts to a concatenated DiscourseDiscovery User Page (with the option to leave out any they want to). Badges can be tallied here, as well as latest activity, etc, etc.
Forums could also have the options in Security to restrict access to certain things (categories, polls, etc) to full members only. They could also choose whether to be included in the public directory, or a more private one based on a user’s Hub_Levels (or to not be included at all).
Just some ideas.