I'm looking forward to all the deviant guilds and seeing how long they last and how well they perform. I'm sure the nobility will have their own personal spies and whatnot, but a large and successful thieves guild that the authority struggles against or can't do anything against would be cool. It's hard to count how many fantasy worlds I've read about that had thieves guilds in/under the larger cities. They're always the more interesting of the guilds too. I mean, mage guilds are cool, but it's different. Fighters guilds are.... simple and bland.
The thieves guilds in the elder scrolls games are a solid example. With the identity system in CoE it's quite possible to have a "Gray Fox".