I was reading The Da Vinci Code and got an idea when I was reading about the Knights Templar in real history. They became a really powerful guild and extended their credits to people, while charging interest. Then I remembered playing Civilization IV, where you'd need to learn the technology "Guild" before you can discover "Banking".
Since Guild Wars is focused on guilds, we should really exploit that idea. Factions already touched upon the idea of spreading a the power of a guild by having them own outposts. It'd be nice if we take that further.
Guilds will need a certain amount of money to start a banking business. They can loan credit to players in need of money and charge interest by a certain rate. The money will probably be stored in the guild vault and maybe that money can be used to develop the guild and its business.
For example, a certain guild leader will need a lot of money to buy a guild hall, or hire merchants, or start a business in the outpost, so he'll need to take out a loan from another guild. He'll be charged interest (weekly or daily) and will have responsibility of paying back the principal.
Pretty soon, I think guilds should be able to start all kinds of businesses in the outposts they own. They could hire merchants in their outposts to sell weapons, craft items, and such in their guild vault.
I'd like to see the true potential of guilds in this game


(Delete their character) And poof the guild that just loaned would be scammed. I think a more NPC related situation would be better. Such as having a material income... You could buy plots of land say... In the Shiverpeaks and mine iron ignots, steel ignots... etc etc. Then be able to make a profit off of that. Or buy a plot in Kryta or Maguuma Stade for lumber to get wood planks. Putting a tax on items sold from merchants in the town you own would be cool also. The money would go to the guild vault which would be evenly distributed amongst all the guild members.