I think if a developer had made such a reference, it would've made the news here at EN World. I've certainly heard no such thing. As far as I'm aware, Princes of the Apocalypse is a standalone product. The next adventure will be about something else.
That being said, it's possible what you saw was a reference to an upcoming Adventurers League module for the Elemental Evil season. They haven't announced the titles for all the D&D Expeditions adventures, nor have they announced a D&D Epics adventure for the season either, I don't think. Maybe that's what you saw?
EDIT: Or could it perhaps be the (working) title of an upcoming novel for the Elemental Evil storyline?