"Ours is a cycle of hatred, alliances forged and broken. We have paid the price for sharing this world, and we have forgotten what makes us strong"
Here, at the very beginning of the expansion, she's basically talking about union. Horde and Alliance united as one to fight a common enemy, thats what makes them strong. Without a common enemy to fight, they either bicker amongst themselves or go lazy. She then gives them a common enemy to fight while the real big bad doesn't show himself: herself. 8.2.5 gives you a title, Veteran of the Forth War, to show you that she was succesful, and the war among factions is over. Every step of the way she has been working for this.
1) She kickstarts a war with the burning of teldrassil to end the lukewarm tension and get the juices flowing.
2) She has us pick up Talanji at Stormwind while deliberately leaving Saurfang there, aware that he might cook up a revolution.
3) She keeps the tension high while instigating said revolution with Baine when she ressurected Jaina´s brother and wanted to use him to get to Jaina.
4) She then sends assassins after Saurfang and Thrall to antagonize them. Then, pratically announces that Baine is in chains and will be executed in front of all leaders and alliance spies to get them to try and free him while working together in the process.
5) Once the revolution was formed she led everyone to azshara to force the horde and alliance to work together against azshara and n´zoth.
6) When Saurfang knocked on her door with the remaining forces, she killed him to preserve her image as a nemesis, while allowing the horde to unify itself again and end the war with the alliance. She KNEW she made him a martyr.
Everything she did was with a purpose. If you´re a loyalist, Nathanos constantly tells you that everything is going exactly how she wants it to happen. She was preparing us for something and it obviously isn´t N´Zoth. If she is being forced to work for a powerful entity, she slipped in these actions while keeping in character to have us prepared to face them, while not letting anyone know she was in reality working for good. If 9.0 is the leaked Shadowlands expansion or not, this expansion was all about Sylvanas preparing us for what is to come.
"you are all nothing"
"nothing lasts"
This is too deliberate to not mean anything. Plus the obvious Blizzcon image with her at the centerfold.
TL;DR: Major plot twist incoming: Sylvanas was good all along and was preparing us the face the God of Death even while being forced to do his bidding (i.e. kill people)