Plenty of ways to add diversity. I don’t know what lore they have planned, so I’m not sure how unconventional I can be here, but here’s some simple suggestions off the top of my head that seem to fit what we’ve seen so far: magical artefact, restless spirits of the deceased out for revenge, necromancer cabal, vampire lord, ancient curse, magical plague, the spirit of a great king whose burial mound was disturbed, ancient hero who cannot accept his death and wants to conquer the world of the living, a crypt whose dead can find no rest, old Roman outpost carrying out its last duty, magical ritual gone awry or ghoul elder leads the hunt.
