I guess I'm more used to singleplayer RPG's, and guilds composed of NPC's, but i've always liked the idea, and I believe it is closer to the traditional concept of a guild, than to modern MMORPG guilds, that is; guilds that have headquarters all over the country/world, members are welcome in any hall, no matter where, simply because they are a guild-fellow.
You don't need to know them, you don't need to be friends with them; they are a member of the guild, that's enough.
I have actually (thanks to the new city claiming system ) been thinking of doing exactly this; a guild whose members are spread across the three worlds, who dwell in every city, are members of every race, united under one banner.
A guild without a home, no city shares their name, but members aid each other when in need, no matter race or allignment.