if game would be easily moddable, it would be great if "main lobby" would be somewhat more closer to virtual realm. Then new servers would add a portal or door or something to main lobby. Then after going from a certain door or similar would player connect to a server which would provide some kind of game arena. Player would bring it's characters model, but server would provide armatures so that if server holder wants, he could create a tennis or wrestling arena.
So basically I'm thinking of realm which would be quite close to Cyberpunks vision of internet. Every server would hold it's part of university and there would be connections from server to server.
Main lobbys realm could be served as a bit crippled compared to actual game servers so that it could support larger amount of players. For example network tick rate could be lowered to closer the state it was in time of 28kb modems.
Chaining of lobbys from those "mounting points" which i described as doors or portals would provide possibility to hold smaller amount of players per server and possibility to direct players to area of their interest.
Lets say that there would be athletic lobby with soccer and golf and rpg lobby with mutant rpg and alien rpg.
Those could be brought to game while story evolves. First players would have to kill mutants and aliens to make room for their athletic fields and so on.