I don't think they run in Waterdeep, but rather run out of Skullport which has portals to various places. I also don't think they thieve as much as they engage in slavery.
I'm anticipating a magic item book next fall. More artifacts, more generic wondrous items, more potions and scrolls, perhaps some crafting rules. Possibly the artificer.
So it could be anyone. Elminster's Magical Wonders. Bigby's Bazaar. Book of Artifacts. Mordenkeinan's Marketplace.
Well, Xanathar's seems as much DMG II, and maybe more.Volo's Guide = Monster Manual II
Xanathar's Guide = Player's Handbook II
So the question is, what does a DMG II look like?
I could see Van Ricthen's Guide to the Undead (something along those lines) being a future book...

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.