James Jacobs (Managing Editor, Dungeon), Tue, Jul 19, 2005, 11:32 AM
There will indeed be a Thrall of Zuggtmoy in the next Demonomicon, along with some of her classic minions (and a new one). Her relationship with Tharizdun/EEG/Iuz/etc. will be discussed as well. Not sure what holes need filling, but any I find will certainly be filled in with nutrient-rich soil good for growing mushrooms in.
As for the series as a whole, it'll be sticking wtih demons. It's not called the Devilnomicon, after all! ;-) Also, the theory is to focus on the demons that haven't yet had official 3rd edition entries, so the ones that got covered in the Book of Vile Darkness won't have entries any time soon. We've still got Kostchtchie and Baphomet for starters, and there's that neat list on page 35 of the 1st edition Monster Manual II that's ripe for mining. Chances are fairly good that the next 2 will be Kostchtchie and Baphomet, but who knows? I might get distracted by Malcanthet or Ugudenk or Socothbenoth or Obox-ob or something...