As I understand it around 5% of all sales goes back to the guild from the stores. Where does the other part of the tax go? Well obviously it's just deleted, but wouldn't be fun if it went into an NPC bank, or vault. Then on say a monthly basis this vault could be a raid that guilds could try to take on to get the money back. The "bosses" could all be guard type enemies (so players can finally kill them pesky guards).
Not sure if this should be a timed trial and at the end of the month, or if it was only open to be "raided" over like 3 days the 3 groups that did the trial the fastest get the gold. 1st would get the biggest cut 2nd and 3rd would be a little less respectively.