Having a sub-forum for guilds (application required), would be quite a nice feature.
It would get far more people involved in the forums (community involvement is always a good thing), and allow an easy and effective way of guild communications.
It would allow better discussions across multiple timezones. Something which can be quite difficult at present.
The only downside I can see, is there might be an increase in moderation for security\access for players to these forums. Although I'm not sure if this can be delegated to leader\guild moderators so permissions are self-managed.
Perhaps even have it require Gems or in-game resources to 'purchase' the option, to avoid lots of inactive\dead forums not being used.