Have you ever asked yourself: what does your guild specialize in?
well here's a reason to be guild master bestow on guild a specialization which would grant passive abilities but the catch is changing it around would cost a fortune to prevent constant switching (barring maybe the first few times) to give each guild a more unique feel. What do I mean by specialization? here are some examples:
Woodcutters guild: reduces wood cost and increased rarity chance on all wood based gear
Warriors guild: heroes and champions get bonus exp gains
Alchemists guild: reduced fusion time and increased exp gains for all fusions
Elementalist guild- more income from gear with elements and increase rarity chance from all element crafting
These are but some examples if you want I can describe more. hope you can give your opinion on it