Thinking of the trust levels system for users, I would extrapolate to identify friendly communities, maybe based on the overlap of users and cross-linking between them. I can’t find an example of bad-behaving communities (and please, don’t tell me any) but I guess a hub would be able to correlate like-minded communities…
In some future, when one email would enable users to connect to many Discourse instances, the hub might also become a way to keep a public user profile that a newly joined community would tap into to create the user’s profile without bothering her. In some cases though, users might want to keep their accounts and participation separate, as in work/home dichotomy, and this should also be supported. But maybe this goes way beyond what a simple hub would mean.