Guild halls should have a tier upgrade.
For example:
Tier 1 – Guild Hall
Tier 2 – Guild Keep
Tier 3 – Guild Castle
….
In Guild Hall must be a NPC, who takes a contributions from guildmates to upgrade tier.
For example: For upgrade from tier 1 to tier 2 needs 10k badges from each Dungeon, 5k Badges of Honor, 100k Influence, etc.
Resourses for upgrades must be getting in different aspects of game – WvWvW, PvE, PvP.
And upgrades must not give a powerfull bonuses to guild, just a cosmetic changes.
You can also add a Legendary tier with a ton of resourses for upgrade