While I wouldn't exactly call them supernaturals, I'd like to have robots. And not just one male and one female version either, as it was with Servos; I want to be able to customise them just like human sims, decide how human they will look, if their voices have an echo to them or not, what material ("skin") they're made of, etc.
As a life state, they would also have several strenghts and weaknesses and unique gameplay options; I would find it interesting for them to come with a limited capacity for skills, for example, so they can learn every skill faster than humans, but only master so and so many of them before their memory is full. If they want to learn something beyond that, they have to either delete something else or be upgraded somehow. Speaking of upgrading, a new robotics skill should definitely come with whatever pack introduces robots, and robots shouldn't be waterproof until the corresponding upgrade has been done.
The first sim I would create with a feature like this would be EDI from the Mass Effect series, and now that I think of it, I wish we already had it...