While a good majority of forums are open and public, some are more private and seek to be built around a small community.
I was wondering [if not in XF, than as an idea for a plugin] about the following:
Rather than letting any user register however many accounts they want on a forum, how about an option to allow only current users to invite new users to join the community? Through this we can see an invitation tree from new user Z who was invited by user Y. We also can see that user Y was invited by user X.
Creating an account based off of invitations rather than open registration is a little bit different, I admit. However it's also a pretty neat idea for niche communities, such as video game clans and guilds, as well as comic shop table top board players, and the like. It also frees up the admin from having to check each new user registration, as invitation cuts out a most chances of spam bots or unknown weirdos joining a forum they weren't invited to.