My suggestion is, she would have kept the war going, manipulating both the Horde and the Alliance to keep fighting.
Since we never saw a great number of banshees in Sylvanas ranks, even though in WC3 lore the banshees were actually the only ones who really stood loyal to Sylvanas and were the only troops she could even build herself, but now that shes in her hideout, its suddendly full of banshees, I belive stronger than before that she used her banshee sisters and their ability to possess living beings in her manipulations alot. The banshees were obviously still there, so why hide them from the horde if you are not using them against the horde?
So, the war goes on, and in the long run, since both sides would run out of warriors sooner or later and the horde civilians are more capable of being recruited based on their superior strength and endurance by nature, The Horde would have “Won” eventually.
The moment the last Alliance defender fell and all military threat was eradicated, while the soldiers who fought for their Warchief were still celebrating victory, Sylvanas would have her loyalists execute her version of “Order 66”, killing every Horde warrior still standing. Or maybe just blight them all.
After that, since she would have made sure to keep her own loyal soldiers save and she would have just ressurected the fallen as forsaken slaves, she would just spread death all across Azeroth, until no living soul remains.
After that she would have gotten rid of every Forsaken she cant control 100%. Including the players loyal to her, reaching her end goal of destroying everything that could be a threat to her life.
looking at where she is now, ghostlands it seems like, I would not be to supriced if silvermoon falls soon, specialy as it looks like we might get a faction merge and maby a new faction.
I think the outcome may still work in Sylvanas favor, the Horde and the Alliance now banding together and fighting N’zoth. The fighting will undeniably result in a lot of casualties, which Sylvanas desires.
If the new union between the two factions manage to defeat N’zoth and hold a celebration after the victory. There’s one certain faction blinded by hatred who will be sickened by the sight of Alliance and Horde cuddling together.
So what if the Night Elves prepares for a “red wedding”-esque of attack on the Horde and the Alliance while they let their guard down?
At that point, it doesn’t matter if the massacre fails or if the Night Elves manages to slaughter everyone. The outcome will be like christmas for Sylvanas and her loyalists.
She can enjoy the chaos from afar while eating a medium raw Tauren steak while sipping on fresh Worgen blood.
Undercity is filled with the Blight that was designed to kill both living and the Undead. I don’t think it’s the perfect place to hide an army.
It doesn’t look very bright for Undercity. The entire ground was blighted where the entrance is, so it will undeniably spread to the lower level of Undercity.