What if instead of clients and relays opting in, just users can opt in?

Sure, but I don't see why introduce this unnecessary complexity. In most cases as a client you can pretty much - for whoever you follow or are interested in, request data from any relay, whatever relay on Earth, you really don't care what relay - for cases where you'll get data from people you don'...

I think you're both right, but I tend to lean towards the FoaF (friend of a friend) moderation algorithm. FoaF makes newcomers work to get reach, because they need to know someone, and it's something that Scuttlebot did right so we all know how it works. You can include a dial for less toleratant ...