Ok, so Sylvanas feels betrayed by her sisters and the world. She fears one thing and that thing is death. She's lost the Horde, and more importantly, she's lost the Forsaken. She's also lost, or at least lost faith in, the valkyries.
What if her "new magic" is not new? What if it's a new twist on something old? Literal shadow weaving magic. Between light and void. Different enough that neither Jaina, Thalyssra or Lor'themar could detect it. It is said in the game that Sylvanas sent the Forsaken out on all kinds of missions to find new sources of magic to empower her to escape death. The Shadow Ascendant sounds exactly like what Sylvanas is becoming. And she seems to be struggling to contain her shadowform in Mak'gora. She seems to be delving into things shadow priests are taught to stay away from.
What if Sylvanas happened upon The Forgotten Shadow cult?
Could she mass resurrect a new faction? The Forgotten Shadow?
Could she actually be good guy Sylvanas, trying to remake Azeroth into undead, immune or at least resistant to the old gods magics?
Is this what the old gods fear, and why they may be trying to use us?
Could "the child of light and shadow that will bring an end to the war" be Sylvanas rather than Illidan? (who refused the Naaru)
The undead as the enemy of all? Perhaps Bolvar stepping in to balance undeath and raising new death knights from all who fell in the fourth war to fight against Sylvanas corrupted shadow faction in 9.0?