With all the Arthas Imagery in BFA cutscenes, I thought Lich Queen Sylvanas was where they were going, namely when she had Xal'atath, a weapon that can hold souls & have little blue runes on it. But then she gave the Blade to Azshara, who gives it to your party to kill N'zoth.
Perhaps we'll get a reverse Wrath of the Lich King Expansion, where Bolvar aids the Alliance & Sylvanas aids the Horde in an unofficial capacity, granting us necromatic powers against whatever the next existential threat is? Perhaps the Fascist Xerite Lightbound?