This could certainly be a good way for a small set to have a big impact on the game. It would be cool if there was a stack of 12 unique Guild cards which give different passive abilities to their members. At the beginning of the game, each player is dealt two at random, choosing one to keep. It would also be interesting if each guild had a certain Kingdom pile attached to it which was automatically in the game, and the guild member with whom it is associated got a discount on that card. This would be really difficult to balance, but it could really add some variance to the game, forcing people to try strategies that might otherwise be ignored.