Some things that IMO would be good additions to the actual game itself to support guilds:
(1) GuildLeader and GuildMember account attributes (similar to the chatmod attribute) for each guild.
(2) Until we have a big fancy GUI for guilds, new chat commands for the leaders: "/guildadd $name" and "/guildrm $name". /guildrm removes the GuildMember attribute from anyone in their guild. /guildadd sends a message to $name stating that they have been invited to $guild. New chat commands for everybody: "/guildaccept $guild" and "/guilddeny $guild". /guildaccept (when they have been invited) sets their membership in $guild (which they have to name), /guilddeny or logging off clear invited status.
(3) Have chat/who show [Ca], [Sp], [Ma] as a prefix for guilded people. Leaders can have <Ca>, <Sp>, <Ma> instead.
(4) Add a "Guilds Information" button. It should display all of the following on one screen, assuming there's enough screen real estate:
-Leader and roster for each guild
-Leaderboard showing total wealth of all members in the guild
-Leaderboard showing total combined pve wins/losses of all members in guild (as shown on members' accounts, regardless of when they occurred) sorted by count of wins
-Leaderboard showing total combined pvp wins/losses of all members in guild (as shown on members' accounts, regardless of when they occurred) sorted by count of wins
-Leaderboard showing total guild pvp wins/losses for the guild. Whenever a person from a guild pvps someone from another guild at any time for any reason, this counter is incremented for the guild. It is stored serverside by guild, and only counts things from a member who actively has a guild attribute.
Probably some other things I'm forgetting right now.
PS if you do all of this, it'd be yet another good time to add pve leaderboards, pve competitions, and all the other leaderboards i always whine about.