We've datamined quest names possibly tied to a new Xal'atath questline in Patch 10.2.7, Hunt for the Harbinger. While this quest is not testable on the PTR, we can see the names of the steps and glean some hints about the storyline.
Notably, we see that this quest series will take us back to Telogrus Rift, the starting area for the Void Elves--a race that has been relatively underdeveloped in lore. We can see on the PTR that Telogrus Rift received visual updates, which will likely play a role in the Xal'atath quest Defend Telogrus Rift.
At BlizzCon we learned that Alleria Windrunner and Xalatath will have a «unique rivalry»--which appears to start with this 10.2.7 questline as Alleria begins investigating the identity of the Harbinger. We may have even gotten a few glimpses of this questline in the War Within Features Trailer, which showed Alleria traveling to Dalaran, intercut with whispers and visions of Xal'atath.
Xal'atath's new War Within model has also popped up in our 10.2.7 data, so hopefully we'll get our first look at her revamped appearance in this new Dark Heart questline!
If you'd like to learn more about these two compelling characters, check out our lore deep-dives:
Xal'atath's Unique Rivalry with Alleria WindrunnerGetting to Know Xal'atath and Alleria Windrunner