Suggested histories of the guild. In Gangs of New York you were clued in a little bit about how these gangs were begun, and for what reasons. If given a starting point, then the DM can more easily flavor the current guild; the PCs might see opulence, but below that is the thuggish origin of the place that sometimes shows its head. Also reminds you how ruthless criminal lords must be.
Information on the small-timers. It seems to me that all Thieves Guild stuff seems to rotate on big, over-arching, 500+ member guilds (the Triads, the Families, the Smuggling Cartels, etc.). I want to hear about the three thieving pals who obsensibly make their living cleaning chimneys, while cat-burgularing at night. Or the n'er-do-well horse rustler. Give the small, unnoticible, under-the-radar guys a chance in the spotlight. After all, the less visible you are, the less likely you are to be caught, eh?
Something on the economics of a thieves guild. Under what conditions do they grow? What conditions drive them out? When does a thieves guild (instead of a bunch of independents) make good economic sense?
An editorial (completely your opinion) on where thieves guilds (whose profession is to take what is not theirs) lie on the Good-Evil/Law-Chaos axis. While this opinion might not be subscribed to completely, a clear definiton of the profession would help to enlighten each DM's opinion.