Personally I think that if there was like a central link/front end to access everything and then have each user be able to have a recommended list of instances/hosts to access would be a more user friendly solution and a better solution for search engines. I know the fediverse is about decentralization but having a central front end and decentralized back end seems easier for new users. And then for the back ended hosting aspect each host would be able to manually pick which instances or communities to host and mirror. I'd like to hear your ideas since I have no idea what I'm talking about.
Well, since you don't mention boundaries, it could be very different to todays platforms. For example every 'instance' is just the frontend/entrance to the same, decentral network that stores the data. Your username would work on every single one of them and it didn't really matter which one you picked.
We could also do away with the moderators and have it a democratic place. We could either all vote on decisions or delegate our power to other individuals and keep something like moderators/sheriffs but elect them democratically.