create some set of rules for “default instances” - every instance that wants to be in the list must follow them and will be periodically checked

I don’t have any particular rules in mind, but some examples might include active moderation team, obviously registrations being open and if you really want to make it easy, either no application question or having it automatically approved by an automod of some kind

The Mastodon Server Covenant is pretty much what you describe here, and could be used as a starting point: joinmastodon.org/covenant