This is the part I'm struggling with. I can think of a million things she might want to do with the Old Gods' blood, but I can't decide which makes the most sense:
- To corrupt the worldsoul
- To corrupt the Hallowfall crystal
- To empower/resurrect her old body
- To empower her current body
- To corrupt the Sunwell
- To corrupt the Sword of Sargeras
- To feed to the vessel of Galakrond's essence
Any number of them could serve the purpose of either empowering Xal'atath herself, creating a vessel for another being (Dimensius, Galakrond, the Old Gods, a new Old God, another Void Lord, some other character who "knows our true power" or whatever the quote was), or opening a portal for someone.
I have a feeling it's not the crystal in Hallowfall just because that's going to be central to the zone story in Hallowfall proper, and those sorts of things usually don't then play into the larger narrative, but who knows. I WANT the crystal in Hallowfall to be the core of a sun and to be used as a vessel for Dimensius to enter reality without his body dissolving.
The most likely to me is that she's going to pour the blood into the Sunwell to re-darken M'uru's heart and create a vessel/portal for Dimensius to enter ("a dark heart left broken awaits the taking"). They seem to want the stories of the expansions to connect, and that makes the most sense for me based on the information we have.