I suspect either the following:
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It’s been datamined that we return to the seat of the Arbiter during the Sylvanas encounter, the place where she allocates souls to a particular afterlife. I’m imagining that we fight Sylvanas to the point that she is almost defeated and she escapes into one of the infinite portals to varying realms
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Tyrande will have some involvement. She is hell-bent on killing Sylvanas for what she did at Teldrassil, so for her not to land the killing blow, or to get some form of vengeance, would mean her entire story from Teldrassil amounting to nothing. We’re yet to see the true extend of the power she has as Night Warrior; she has the essense of an actual god flowing through her veins… surely we need to see just how strong this power is?
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During the fight, Sylvanas is slowly defeated, and she seeks the Jailor for help, only to realise she has been a puppet to him all along. I’m imagining she cries out for help, and he ignores her, because “every soul has a purpose.” Sylvanas purpose is to distract the heroes. She see’s this, and thus she changes sides
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She dies. We must remember that Sylvanas is still mortal. She died when she flung herself from Icecrown, but her soul was returned to the realm of the living as part of her arrangement with the Jailer. So IF she dies in the upcoming raid, her soul will just return to the Maw, albiet with none of her banshee powers. I am imagining that, further into the expansion, Sylvanas was privy to some “super secretive” information to defeat the Jailer, so we must venture into Torghast to find Sylvanas’ soul and get her help in defeating him.