The guild-leveling idea is alright, but honestly I don't see a huge relevance nor a benefit to STS that would make it merit the resources to do that. However, if you said instead that they could create a guild-specific vanity of some sort, as many MMO's have, where the guild master can pay, let's say 100 plat, with some non-basic options for additional plat create a color-scheme based off of a few simple options to in addition to the Tier3 guildhall's 100 plat or even include it in the price of a possible Tier4+ guild hall. Then each individual member could pay another 5-15 plat for the duplicated version, or even have a slight difference/graphic to represent Officers, Recruiters, etc. which you could charge a little more for.

For example, simple options for a vanity armor:
Six different bases for the appearance (maybe even have non-basic cost a little more plat on a +5-10-15-20 scale)
Six different base colors (maybe with a similar non-basic colors = +5-10-15, etc.)
Six different trim colors (same...)
Six different options for a graphic on the chest or some kind of flair/spell effect (you could charge primo additional for these +25, 50, etc)

With this you get 1296+ potential options for the vanity item, and STS can rake in 100-300+ plat for the initial creation, then get another 5-20 plat per member that buys one. It's a simple thing to do with a relatively easy code to create, and a proven success rate in other games that could generate an average 10-20 plat per guild member in the game... and I'm guessing there is thousands.