There were spooky-dooky hints about Tharizdun in Princes of the Apocalypse, but it never quite went all the way in detail.They've been hinting at Tharizdun for awhile. One of them mentioned or at least supported the observation recently that Tharizdun was the ultimate D&D villain.
I mean, Wizards of the Coast paid him a big wad of straight cash money over 20 years ago to settle all outstanding issues, so that's not a live problem.Considering how TSR screwed over Gary Gygax I wouldn't feel right for a Tharizdun mention without there being a straight statement that they were doing it to honor him and make amends for how their previous owners treated him.
Well, there was "The Styes" that they could redo for 5e..... hey, wait a minute!1. Do a Ghosts of Saltmarsh style reprint with the Temple of Elemental Evil, Lost Caverns of Tsjocanth, and Temple of Tharizdun (were there any Dungeon magazine Adventures tied to the Elder Elemental Eye/Tharizdun?)