It wouldn’t be pulling in data yet from other instances, but it would enable seeing that their communities were out there

It would have to pull some data, but it could be much less than federating every community.

I think it would be great if, as soon as two servers become aware of each others existence, they automatically federate each others lists of communities, and possibly the sidebar info.

Federating the rest of the data would happen only if someone subscribes to a community from another instance, same as it is now.