Well, we've got guilds, we've sort of got sets of gear, and several classes, now the next level would be mixxing all of those together?
This will help enhance the idea of exclusive guilds, because nobody wants some newb stealing all the best drops from a boss, and people will pretty much be getting friendly and want no outsiders screwing with them.
For players to create their own exclusive guilds, there must be some "non-exclusive" guilds aswell.
Guilds should not have a maximum number of players, if some moron(pardon my french) decides to invite 200 people into 1 guild, then he will be screwing himself over by the end of the day. and this is why:
Raid dungeon:
There are dungeons where players with high skill may enter with their comrades to seek ancient treasures(rare items.
A raid Dungeon would consist of high level monsters & bosses whom hit harder than usual and endure more damage, but also drop better than usual, there are however several sets of drops, one which benefits each class, so you will have to wait until your set item drops by clearing the dungeon several times.
There is room for 16 players within a raid dungeon.
A raid would be like starting a guild-sized party otherwise known as Raid ingame, a player would have to join another leading him to a raid, possible indication r/lead <Bob is inviting players to raid> to join then r/join
Party dungeon: basically the same as a raid dungeon only on a smaller scale, 8 players, monsters hit harder than normal monsters and endure more damage but they drop uncommon items.
So by the end of the day, the game is about getting gear and cool stuff, so people will actually make their guild more exclusive so they can get their gear faster, and this will probably make people apply to guilds formally.