Could the hub also be an opt-in public directory of Discourse sites? This could partially answer “what’s in it for me” for a site admin.
Would it ever be able to aggregate content across Discourse instances? For example, could my Hub profile page show me all the posts I’ve made on different sites? Centralized notifications?
Disqus does something similar, for example https://disqus.com/by/elevatedliving/. Really a lot of what Disqus does there makes more sense for communities rather than what’s primarily a commenting system… (turns out they also have communities, e.g. https://disqus.com/home/channel/starwars/top/).