I‘m also interested in the background of the undead. Are they all created by Necromancers? Think not as Erik wrote that sometimes the dead will not stay dead. Like that idea.
And will there be liches and vampires?
Combining classic necromancy, the ”naturally” restless dead and demonic possibilities, there's really no technical limit. I'm more concerned about putting the undead in context, building interesting and believable scenarios and challenges. Also I should mention that the elves in this setting are a little shady and basically immortal, being able to regenerate from their ruby cores even if chopped to pieces. You might encounter something bad, lich-like down that road as well (I liked the concept of the bored immortal vampires in ”Interview with the vampire”)
So many possibilities and only one book to squeeze them into!