I would like an instance type that allows only friends of the host, and friends of the host's friends.
The main issue a lot of people have with Friends+ rooms is the infinitely chaining friends of friends of friends of friends..... etc that come into the room, turning the room into a lot of strangers that some people just don't want to deal with.
I think this type of joinable instance would be much more popular than friends only or friends+, and would have people join each other's worlds more often by creating less private instances to avoid friends+.