Something like the GTS (which I assume more people would be interested in) would require a globally established connection. So a dedicated server or something which will allow all games to connect to a centralised "station", rather than just a connection between two machines. The battle system is different though, since it only considers two players. But if we had some global communication with that too, you could have things like player tournaments. It depends what kind of interaction you're trying to have. I think a global one would be better, as that way you'd have a master database which would handle everything automatically, rather than users having to manually establish a connection.