Draka and the dead Iron Horde: Undead Orcs
Kaelthas: Undead Elves
Caelia: Undead Human
Derek Proudmoore: Undead Kultiran
Bolvar
Uther
Emperor Shaohao
Voljin
Rastakhan
Cairne Bloodhoof/ Huln Highmountain
The Shadowlands Denizens of Ardenweald, Revendreth, Maldraxus & Bastion
Undead Elves
Undead Human
Undead Kultirans/Kvaldr
The Forsaken
Seems like they can be a cast which can make a new solid "Undead" faction not tied with either Alliance or Horde