I am for a "Heavy Metal" factor in guild rating.
The more members in a guild, the higher the moshpit-factor, and the more often the guild members do stuff together, the more "true" the guild becomes.
And each guild member gets a "true" - factor depending on a totally complex, unlogical and brain-dismantingly buggy formula consisting of the character's online time, the average time spent with group members, the total hour of powergaming the character went through, the time spent killing non-guild player characters, the average number of beer consumed per online hour by the character, the average number of beer consumed per online hour of the player, and the time spent cybering.
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Joke aside. I am for specialized guild items depending on the guild's active member accounts which guild members can buy from an NPC or a GM.
These items must be bought for the actual value their non-customized items have and the items of the same type MUST be identical for each guild member.
For example:
5 members : simple guild item (a uniform hat or a cloak, cheap-ish, with a custom description)
10 members: better guild item (expensive type uniform item instead of the first one)
10 members: guild can get a building.
15 members: second item: guild uniform shield or weapon (simple item)
20 members: second item improved (better weapon or shield, magical)
25 members: guild item with special ability (something like, making sheep-baa or fart noises when used, nothing too powerful like teleports etc.)
25 members: ...and so on