Imagine vanity items specific to that guild. Like statues made of different materials like gold, marble, or plat. They could be images of members lining the walls created by members as reward.
Or howabout armor/weapon racks. This could work like a limited stash that would display sets (w/sparkles, if).
Or trophy mounts on the walls. You know you would pay plat to have one of those oneshotonekill boss' head hanging up mounted.
Dig?