I would much rather prefer the Lich King continuing to be an anti-hero. He saw the devastation of Battle for Azeroth and the oncoming threat of the Void Lords/N'Zoth/Shadowlands and raised more DKs. Therefore he would naturally resurrect EVERYONE as DKs, including Allied Races.
I don't think we want a repeat of "Wrath of the Lich King" instead it would be cool just to have the Lich King as a possible ally. Let Sylvanas be the leader of an undead force that is our opponent instead.
