So to add something special to guilds, can we maybe introduce something that promotes guild play / wearing the tabards. It could be that when the leader creates the tabard they can select a special ability to place on it. It could just follow the Mundus stones but perhaps not as powerful, Increase weapon damage, max stamina, stamina regen, spell damage etc.
Then once a guild claims a keep, once inside the walls of that keep, everyone receives that buff. It'd create another dimension to PVP that could be quite cool even if perhaps the buffs only applied to keep claimed by guilds.