I think outbox defaults can only be properly non-centralising if they are not penalising. person A can't be given a pickier set of relays than person B, such that person A just can't do or experience certain things they want to, whereas person B is fine for those same tasks or experiences. just based on luck of the draw.
unless you have some uniformity in the base set then even for outbox you need to force a choice.
they wouldn't even be given any choice at first, outbox should handle it all for them. the client would look for the relay list of the npub they're trying to follow, in a predefined set (including nip65 specialized relays), which should be as large as possible.
once the relay list is retrieved, thats a bunch of relays to explore, and in those relays, they might find hundreds of relay hints in events. thats how the magic happens, unless they're exploring nip70 dedicated relays.
now if their point of entry is a relay, it's either:
- amazing, a regular relay that relays events as it should
- terrible, that relay is nip70 only, all the events are exclusive to it, no other relay hint can be found
