So, with Sylvanas being the last encounter in the Sanctum of Domination, I think it's safe to assume there will be a cinematic after we defeat her. It's too important character to just die or escape and for the encounter to end anti-climatically. Her placement within the tier structure also implies there's going to be some big lore reveal, since most if not all end bosses are very important in pushing the plot forward.
Even before Shadowlands released there's been guessing if Sylvanas is truly evil, or if there's a greater picture we fail to understand. She speaks about the cycle she and the Jailer want to break, she's seen things she doesn't want to talk about. Devos during Spires of Ascencion says that she's learned of the great injustice thrown upon the Jailer, but also upon the whole Shadowlands.
Looking at Sylvanas short encounter journal entry:
What we'll see there? Keep in mind that, for now, we've seen a very limited portion of the Shadowlands. We were allowed to see only what the Fatescribes wanted us to see, nothing more. The whole of Shadowlands so far seems a bit too structured, to ordered than I'd imagine.
Do you think that, at the end of Sylvanas encounter, at the Convergence of the Shadowlands, she'll reveal the truth (if there is any) to us? Or is she going to learn of that truth herself, realizing that she's been a pawn?
The way I imagine it is that we've been lead by the Oribos attendants the way they wanted us to use, but not for a good reason, just for their reason. That the scope of the Shadowlands and its mysteries makes it hard for us, mortal beings, to fully understand, and at the Convergance we'll be able to witness the Shadowlands in its entirety, or maybe even beyond that, and we'll realize there's no good or bad side.