What if OGS, KGS, Tygem, and whatever other go server out there were to agree on a protocol that allowed users to play games across servers? How well could that work (from a social, not technical, perspective)? What would it look like? How would you design the user interface, if you were in charge?
(Let’s ignore ranks (i.e., Imaginary Internet Points), for now, because that’s way too hard to think about. It’s difficult to solve that for even one server.)
Interesting idea. And as to how to design the user interface — well, like the UI of OGS, of course 
User interface would be the interface of the server you use, right? A KGS player would indicate that his challenge was for “all servers” for example, and the challenge would appear on OGS with some kind of indicator that the opponent was playing on KGS rather than OGS.
Yeah, that’s what I’m envisioning.
Some servers allow negotiating time controls, handicap, etc. That might just require some sort of counter-challenge system.
It would also be cool to be able to send a direct challenge to someone else on another server.
We’d actually proposed doing something like this with Kaya back n the day, but the effort vs reward was laughable then. (In reality, nova.gs was just getting started so of course we were interested, but what would Kaya have to gain from such a thing? nada). In general I suspect this would always be the case, the smaller servers have everything to gain and the larger servers don’t really gain anything and stand to even lose. The #1 “feature” of a Go server is its community, people develop friendships and rivalries on servers and get rooted to that server even if that server starts lacking in features in comparison to other servers. By introducing cross-server protocols, you’ve suddenly removed the downside for a player to switch to a different server since now it doesn’t matter where they play, they can still play with their friends and rivals.
That being said, if such a thing were to gain momentum with any of the other major servers we’d certainly be interested in linking in 
(But it’s not something we’re gonna do a build-it-and-see-if-they-come though, it’s a whole lot of work for what I suspect would be no return…)
Yeah, that’s what I also thought, @anoek.
I also think it would be quite difficult … imagine a third-party app that is polyglot, i.e. understands all protocols from all servers … and then imagine two people playing, one on KGS and the other on IGS — that same game would have to exist on both servers, no? So also the players would have to have accounts on both servers … OK, so then we have to imagine that also … so they are playing and come to the end … both would have to negotiate removed prisoners, etc., on both servers, or do I get something wrong here?
From the users point of view, this would just look like a single server with multiple different clients. From a technical point of view that is almost what it would be. It would be a bit like IRC, where you connect to a network instead of a server. Behind the scenes there are multiple servers, but from the users point of view he is just “on the network”.
If the established GO servers were to form a single GO network, that would probably have some social consequences seeing as the different user bases would be collapsed. We have already seen this in small scale with Nova and OGS.
Anyways, some open protocols in the GO would would be cool instead of the current situation were all the servers are using their own private protocols except for IGS, whose protocol is a million years old and without documentation.