Perhaps an interesting addition to the theives guild could be special "burglary" missions that you can only do if you are a member of the theives guild. They could be like town board tasks that you can only get from a special NPC in the theives guild. The mission could be that a special house/mansion location is randomly generated on the world map and the theives guild NPC tasks you with either stealing all lootable items on the map, stealing a small number of specific items that the NPC requests or stealing one highly valuable item somewhere in the house/mansion and bringing it back to him.
This randomly generated map could be populated with hostile NPC's that attack you as soon as they spot you (because your trespassing and robbing them). You can choose to eliminate them all if you wish or bypass them all and just rush to the items and quickly escape. NPC's can be high level compaired to your character to promote avoiding them.
When you return to the theives guild NPC and talk to it, it will take all the stolen items from you and pay you back in an amount of platnum coins that is larger than that of town board tasks. Perhaps you can even get a bonus of somekind if you didn't kill any of the NPC's on the map when you robbed them. This mission can be repeatable of course.
Something exclusive to the theives guild like this will definately create more incentive to actually join the guild so they can play the mission that you can't do otherwise. Besides, who dousn't like more maps to explore?