I think it could be an interesting theme for a full expansion that goes full into the occult like the supernatural expansion did for Sims 3, but with limitations. Not all combinations would be possible.
Obviously all could be a spellcaster since it's even silly this is listed as supernatural.
But other than this, they could have like a Frankenstein monster that's new, an undead flesh golem of sorts I suppose, with maybe a whole new occult science skill. Your sim could get to take on the role of Dr Frankenstein.
The robot could be revised as well, actually getting different appearances possible, just like the new flesh golem.
Cyborgs could also become a new possibility, a way to mix robot with some other occult states.
It could be incompatible with mermaids due to needing water and werewolves due to the transformation, but it could work fine with spellcasters, aliens, vampires and flesh golems.
Werewolves could mix with vampires, they would get some traits from both and when turning they would become sort of a vampire werewolf creature, looking undead with sharper teeth, claws and vampire eye styles, maybe walking like vampires too or a new combined walk.
Werewolves need actually scary options anyway, I never saw less threatening werewolves in anything else.
A werewolf alien could look like some odd furry alien being, or maybe even not have any fur, like it affects aliens differently.
A vampiric mermaid could be pretty interesting, preferring to swim in the dark and maybe a new coffin filled with water could be available to them as a bed. They could be capable of getting blood from fish while swimming.