So many things could happen, it will be an organic process of evolution. Let's say communities end up consolidating (e.g. one of the gaming communities becomes the prevalent one), then the server that has the super successful community, how will that affect it? Will people want to be on that server because of that community? Will people prefer to use a server that stays topic-agnostic but maybe offers a strong identity (vibe, political leaning, geographic, etc etc) and that's what attracts users?

Maybe those servers don't even have any of the usual suspect of communities, because it's expected the users will go subscribe to the popular one. Maybe it only offers local info threads, or open format / ideology guided discussions.

Or on the other hand will we see more and more specialized servers that are focused around a set of communities from a specific field?

I guess I'm just wondering how it will all fall into place after there is massive growth, it'll definitely be an interesting part of human behavior to observe.

cross-instance authentication

Yeah that is a big one indeed, I was very confused at first why clicking on some links wouldn't work how I expected it to until I realized there were some hard links that would take you to that server's page directly, and since you're not a user there ofc you're not logged in. That could be better.