Sword of Spirit
Legend
Also the clues to the next thing are in the current thing, right, that's what Chris Perkins has hinted at.
That was certainly true with regards to Princes of the Apocalypse/Elemental Evil products. The core books (at least the Monster Manual) had too much mention of "elemental evil" for it not to be hinting at something.
Did Princes of the Apocalypse have similar hints about demons and the Underdark? (I don't have the adventures.)
If so, the best guess would come out of reading carefully through Out of the Abyss, and noticing what seems to get more than its expected amount of mentions. (For best results, apply to a brain saturated in decades of D&D lore.)