Here are some additional ideas for guild improvements, in part based on past thoughts:

Automatically remove inactive members:
After a set amount of inactivity, automatically demote or remove members. If the sole GM is removed or demoted, the next highest ranking guild member is auto-promoted to GM. GMs may set demote/remove, and the time period.
This should be available to all guilds from the get-go with no cost

Guild Coalition:
GMs can send coalition invitations to other guilds (they can click on any member, but only the GMs of the other guild receive social mail about the invitation)
Knights in allied guild coalitions ranked Member or higher can join an allied guild without an invitation. Their displayed guild and guild hall access will change, but they will have a mark indicating that they are a guild ally, not an original member
Guild allies that have joined through this function cannot be promoted or demoted, but an officer or GM may ban them (this will boot them back to their own guild, and prevent them from being able to join that allied guild until an officer or GM unbans them from the guild's Banned list).
This would definitely be a premium guild feature, with each guild in the coalition paying an amount based on the number guilds in the coalition and/or the number of guild members.

Guild Karma/reputation:
Each day, knights ranked member or higher get two karma points to award to other guild members (including recruits); typically, you would award points to someone who has been cool in some way. They can also choose not to distribute any points at all for that day.
Karma/rep points expire exactly one week after they have been awarded. This would be a fun little measure to see who is active and being an awesome person in the guild; high karma could also come with some minor cosmetic perk.
This would probably be a premium feature, but not a particularly expensive one.