My recommendation is in two parts but is very simple.
Add an alliance tab to social where a guild leader can specify up to 6 other guilds and players within those guilds will be listed, to include current level, class breakdown, physical location and button options for sending tells, invite to party; all options similarly found in the /who and current guild social tab. Also, an alliance channel would be added to the chat window to allow open communication between all members of each guild within the alliance.
An alliance request would be sent by guild master's and would appear in the alliance tab as an outstanding invitation with options for the target guild's guild master and/or successor to accept/deny.
This would allow players within both small and large guilds to more easily identify friends who may be available for questing.
Also, a guild alliance would allow the recruitment of "honorary" members. If a player chooses to accept an honorary title, they can toggle 100% of his/her earned renown to be applied to the guild who honors him/her as a close friend or ally. This would not change the honored players current guild association or the guild title text above their character. In reward for the selfless donation of renown, the player would enjoy a +3 honor bonus to the diplomacy skill and a +5 honor bonus to the haggle skill. An honored player can choose to remove his honorary status at any time but will lose the honor bonus to haggle and diplomacy.
Thanks,
Mayhem of Orien